particle-reinforced water-based organic–inorganic nanocomposite coatings for tailored applications

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Particle-reinforced water-based organic–inorganic nanocomposite coatings for tailored applications Brian P. Mosher a , Chunwei Wu a , Tao Sun b , Taofang Zeng a, * a North Carolina State University, Department of Mechanical and Aerospace Engineering, Raleigh, NC 27695, United States b Department of Applied Chemistry, Harbin Institute of Technology, Harbin 150001, China Received 1 December 2004; received in revised form 16 March 2006 Available online 21 July 2006 Abstract We report the synthesis and characterization of three particle-reinforced water-based nanocomposite coatings. The films are sol–gel derived using non-ionic surfactant, with aluminum perchlorate (Al(ClO 4 ) 3 ) as a catalyst and 3-glycidoxypropyltrimethoxysilane (GPTMS) as precursor. Through the aid of nanoparticle colloids and a minute amount of catalyst, dense, hard and monolithic materials are obtained. Incorporating metal oxide nanoparticles brings forth unique properties, such as absorbing harmful UV radiation. Silica colloid composites provide greatly enhanced mechanical properties without modifying the unique optical properties of inorganic mate- rials. Water-based synthesis of these coatings is straightforward and produces very few harmful byproducts, making them ideal materials in industry. The materials presented are relatively hard and abrasion resistant with very good adhesion; two of the coatings are UV absorbent. Various colloids can be employed in our methods to tailor properties and resulting materials may serve applications such as optical, protective, catalytic, guest-host, and multifunctional coatings. Ó 2006 Elsevier B.V. All rights reserved. PACS: 81.05.t; 68.60.p; 68.55.a; 81.20.Fw Keywords: Films and coatings; Nanocomposites; Organic–inorganic hybrids; Sol–gels (xerogels) 1. Introduction The mild reaction conditions of sol–gel processes allow the incorporation of an inorganic component into organic materials, thus it is very favorable for the synthesis of organic–inorganic nanocomposite materials [1]. Under acid catalyst and carefully controlled reaction conditions, trans- parent and monolithic hybrid/composite materials can be obtained through the sol–gel process [2]. However, researchers still strive to produce materials with the unique properties of inorganic compounds which also possess the mechanical properties of organic polymers [1–6]. The relax- ation properties of organic modifiers also provide for low structural densification temperatures relative to inorganic sol–gel materials such as those based on TEOS, enhancing their attractiveness for commercial applications. 3-Glycidoxypropyltrimethoxysilane (GPTMS) based materials are reported to have several applications such as antiscratch coatings [7], passivation layers for microelec- tronics [8], multifunctional coatings [9], and photonic applications serving as host for functional organic molecules [10]. GPTMS, an organically modified silicon alkoxide (ORMOSIL), contains an epoxy ring, which crosslinks to form poly- or oligo-(ethylene oxide) deriva- tives, thus serving as a network former. Ring opening of GPTMS epoxy has been obtained in sol–gel derived organic–inorganic materials with titanium or zirconium alkoxides [6,11] and more recently with BF 3 OEt 2 [12]. Ludox Ò silica particles have been extensively employed in the sol–gel process for synthesis of crack-free mesoporous 0022-3093/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2006.05.026 * Corresponding author. Tel.: +1 919 515 5298; fax: +1 919 515 5934. E-mail address: [email protected] (T. Zeng). www.elsevier.com/locate/jnoncrysol Journal of Non-Crystalline Solids 352 (2006) 3295–3301

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Page 1: Particle-reinforced water-based organic–inorganic nanocomposite coatings for tailored applications

www.elsevier.com/locate/jnoncrysol

Journal of Non-Crystalline Solids 352 (2006) 3295–3301

Particle-reinforced water-based organic–inorganicnanocomposite coatings for tailored applications

Brian P. Mosher a, Chunwei Wu a, Tao Sun b, Taofang Zeng a,*

a North Carolina State University, Department of Mechanical and Aerospace Engineering, Raleigh, NC 27695, United Statesb Department of Applied Chemistry, Harbin Institute of Technology, Harbin 150001, China

Received 1 December 2004; received in revised form 16 March 2006Available online 21 July 2006

Abstract

We report the synthesis and characterization of three particle-reinforced water-based nanocomposite coatings. The films are sol–gelderived using non-ionic surfactant, with aluminum perchlorate (Al(ClO4)3) as a catalyst and 3-glycidoxypropyltrimethoxysilane(GPTMS) as precursor. Through the aid of nanoparticle colloids and a minute amount of catalyst, dense, hard and monolithic materialsare obtained. Incorporating metal oxide nanoparticles brings forth unique properties, such as absorbing harmful UV radiation. Silicacolloid composites provide greatly enhanced mechanical properties without modifying the unique optical properties of inorganic mate-rials. Water-based synthesis of these coatings is straightforward and produces very few harmful byproducts, making them ideal materialsin industry. The materials presented are relatively hard and abrasion resistant with very good adhesion; two of the coatings are UVabsorbent. Various colloids can be employed in our methods to tailor properties and resulting materials may serve applications suchas optical, protective, catalytic, guest-host, and multifunctional coatings.� 2006 Elsevier B.V. All rights reserved.

PACS: 81.05.�t; 68.60.�p; 68.55.�a; 81.20.Fw

Keywords: Films and coatings; Nanocomposites; Organic–inorganic hybrids; Sol–gels (xerogels)

1. Introduction

The mild reaction conditions of sol–gel processes allowthe incorporation of an inorganic component into organicmaterials, thus it is very favorable for the synthesis oforganic–inorganic nanocomposite materials [1]. Under acidcatalyst and carefully controlled reaction conditions, trans-parent and monolithic hybrid/composite materials can beobtained through the sol–gel process [2]. However,researchers still strive to produce materials with the uniqueproperties of inorganic compounds which also possess themechanical properties of organic polymers [1–6]. The relax-ation properties of organic modifiers also provide for low

0022-3093/$ - see front matter � 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.jnoncrysol.2006.05.026

* Corresponding author. Tel.: +1 919 515 5298; fax: +1 919 515 5934.E-mail address: [email protected] (T. Zeng).

structural densification temperatures relative to inorganicsol–gel materials such as those based on TEOS, enhancingtheir attractiveness for commercial applications.

3-Glycidoxypropyltrimethoxysilane (GPTMS) basedmaterials are reported to have several applications suchas antiscratch coatings [7], passivation layers for microelec-tronics [8], multifunctional coatings [9], and photonicapplications serving as host for functional organicmolecules [10]. GPTMS, an organically modified siliconalkoxide (ORMOSIL), contains an epoxy ring, whichcrosslinks to form poly- or oligo-(ethylene oxide) deriva-tives, thus serving as a network former. Ring opening ofGPTMS epoxy has been obtained in sol–gel derivedorganic–inorganic materials with titanium or zirconiumalkoxides [6,11] and more recently with BF3OEt2 [12].

Ludox� silica particles have been extensively employed inthe sol–gel process for synthesis of crack-free mesoporous

Page 2: Particle-reinforced water-based organic–inorganic nanocomposite coatings for tailored applications

Table 1AMicrohardness and adhesion (cured 2 h at 120 �C)

Coating Vickershardness (kgf/mm2)

Standarddeviation

Convertedhardness (GPa)

Adhesion

GPSiO2 45.43 2.32 0.445 4BGPCeZr 143.93 8.58 1.411 5BGPCeO2 96.38 6.39 0.945 5B

Table 1BMicrohardness results (cured 24 h at 120 �C)

Coating Vickershardness (kgf/mm2)

Standarddeviation

Convertedhardness (GPa)

GPSiO2 60.85 3.45 0.597GPCeZr 145.38 0.57 1.426GPCeO2 140.53 4.41 1.378

3296 B.P. Mosher et al. / Journal of Non-Crystalline Solids 352 (2006) 3295–3301

silicates. Most notably is the method developed by Shoup[13], which employs Ludox� colloid mixed with a potassiumsilicate solution. It is known that the particles act as nucle-ation sites for precipitation of silicate polymers and micro-structure and porosity can be controlled by the percentageof Ludox�.

In recent years, federal regulations have become stricteron hazardous air pollutants (HAPs) and volatile organiccompounds (VOCs), creating higher demand for cleanerand safer synthesis of coatings. Water-based coatings cre-ate very low VOCs and virtually no HAPs, making themideal materials in industry. Further, when applying water-based coatings, equipment can be cleaned much more eas-ily, without the use of solvents or other chemicals.

We present a new water based environmentally friendlysynthesis route for the preparation of GPTMS-based, par-ticle-reinforced nanocomposite materials. We introduce theuse of aluminum perchlorate (Al(ClO4)3) for organic/inor-ganic polymerization. Three materials were developed:GPSiO2, GPCeO2 and GPCeZr which are reinforced withcolloidal silica, cerium oxide, and zirconium and ceriumoxides, respectively. The authors have previously devel-oped a method based on GPTMS and Ludox� TMA andshown that the silica colloid acts secondary catalyst andorganic/inorganic polymerization promoter [14]. GPSiO2

synthesis is a one-pot, two step reaction and GPCeO2

and GPCeZr syntheses are one step. The nanoparticle col-loids serve to enhance mechanical properties and also pro-vide unique optical properties such as absorbing UVirradiation. In preparation, a minute amount of catalystis employed to produce a sol which is stable for severalmonths and can easily be polymerized by low temperaturecuring to produce dense, hard and abrasion resistantmonolithic materials.

2. Experimental

2.1. Synthesis of GPSiO2

Materials:

Ludox� TMA Colloidal Silica, Grace Davison, used asreceived.3-Glycidoxypropltrimethoxysilane (GPTMS), ACROSorganics, used as received.Aluminum perchlorate Al(ClO4)3 nonahydrate 99%,ACROS organics, 10 wt% in H2O.BYK-333, BYK-Chemie, 1 wt% in H2O.Zonyl FSO-100 fluorosurfactant, Aldrich, 1 wt% inH2O.Isopropyl alcohol 70%, ACROS organics, used asreceived.Deionized water.

In a typical synthesis, 22.88 g GPTMS and 34.23 gTMA were combined and stirred for 5 min 1.86 gAl(ClO4)3 Æ 9H2O (10 wt% in H2O) and 2.86 g BYK-333

(1 wt% in H2O) were then added to solution and mixedfor 1 h 45 min. Finally, 1.86 g FSO (1 wt% in H2O),7.63 g Isopropyl alcohol and 28.60 g deionized water wereadded and solution was stirred for an additional 5 min.

2.2. Synthesis of GPCeZr

Materials:

Cerium oxide 20% colloidal in H2O, Alfa Aesar, used asreceived.Zirconium oxide 20% colloidal in H2O, Alfa Aesar, usedas received.3-Glycidoxypropltrimethoxysilane (GPTMS), ACROSorganics, used as received.Aluminum perchlorate Nonahydrate 99%, ACROSorganics, 1 wt% in H2O.Zonyl FSO-100 fluorosurfactant , Aldrich, 1 wt% inH2O.Deionized water.

32.67 g Cerium oxide, 32.7 g Zirconium oxide, 26.0 gGPTMS, 4.6 g Al(ClO4)3 Æ 9H2O (1% in H2O) and 3.9 gFSO (1 wt% in H2O) were combined and stirred for10 min. The solution was then diluted 5:3 with H2O.

2.3. Synthesis of GPCeO2

Materials: Materials for coating 3 are identical to coat-ing 2, excluding ZrO2: 65.6 g Cerium Oxide, 26.0 gGPTMS, 4.6 g Al(ClO4)3 Æ 9H2O (1% in H2O) and 3.9 gFSO (1 wt% in H2O) were combined and stirred for 10 min.

All solutions were dip coated on substrates forcharacterization. Substrates were prepared by cleaningwith water, dilute HCl and ethanol. After coatings, filmswere dried for 2 h in ambient conditions and cured at120 �C.

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20

40

60

80

100

0 50 100 150 200

Cycles

Per

cen

t T

ran

smit

tan

ce

UncoatedPolycarbonateGPSiO2

GPCeZr

GPCeO2

Fig. 1. Light transmittance in wear track as a function of wear cycles. Allcoatings show very high light transmission after abrasion compared withuncoated polycarbonate. GPSiO2 shows highest light transmission,therefore highest abrasion resistance.

B.P. Mosher et al. / Journal of Non-Crystalline Solids 352 (2006) 3295–3301 3297

2.4. Characterization

Microhardness, adhesion and abrasion resistance testswere used to characterize the mechanical properties of

Fig. 2. SEM Images at 2000X magnification after 250 wear cycles with 250 g loClearly all coatings show much smoother surface than uncoated polycarbonat

the materials. Microhardness tests were performed on alu-minum 6061 substrates using a Micromet MicrohardnessTester, with a Vickers indenter. At least 10 microhardnessindentations were made on each coating. Adhesion wasperformed on Al 6061 substrate using Gardco Paint Adhe-sion Test Kit. Samples were prepared as outlined in ASTMD 3359, Test Method B – Cross-Cut Tape Test. At least 5cross-cut tape tests were performed on each coating.

Abrasion tests were performed on polycarbonate platesusing a rotary Taber Abrader with a 250 g load. Followingthe Taber test, a Shimadzu UV-2101PC UV-Vis Spectro-photometer (420 nm) was used to determine the percenttransmittance in the wear track area. The wear area wasdirectly observed using a Hitachi S-3200 N scanning elec-tron microscope (SEM). The coatings were applied to poly-carbonate, dried for 2 h followed by curing for 24 h at90 �C. Lower curing temperature was used in preparingthese samples due to thermal properties of polycarbonate.

Fourier transform infrared (FTIR) spectra of coatingsamples were obtained by attenuated total reflectance(ATR) from 4000 to 650 cm�1 using a Nexus 470 FTIRwith Avatar OMNI Sampler using a Ge crystal. X-RayDiffraction (XRD) analysis was performed on cured filmson silica glass microscope slides using a Rigaku D/Max-B X-ray diffractometer (nickel filtered Cu Ka radiation(k = 1.54056 A) under 35 kV and 30 mA). Philips CM12

ad. (A) Uncoated polycarbonate, (B) GPSiO2, (C) GPCeZr, (D) GPCeO2.e after abrasion cycling.

Page 4: Particle-reinforced water-based organic–inorganic nanocomposite coatings for tailored applications

Fig. 3. FTIR spectra of GPSiO2 (A) after drying, (B) after 2 h curing at120 �C.

Fig. 4. FTIR spectra of GPCeZr (A) after drying, (B) after 2 h curing at120 �C.

Fig. 7. TEM of (A) GPSiO2, (B) GPCeZr a

Fig. 5. FTIR spectra of GPCeO2 (A) after drying, (B) after 2 h cure at120 �C.

3298 B.P. Mosher et al. / Journal of Non-Crystalline Solids 352 (2006) 3295–3301

electron microscope operating at 100 kV accelerating volt-age was used for transmission electron microscopy (TEM)imaging. Copper grids (400 mesh) coated with amorphousformvar-carbon film were purchased from SPI Supplies.Films were prepared by dip coating the copper grids fol-lowed by 12 h UV curing under 254 nm UV lamp.

UV absorbance due to metal oxide nanoparticles wasproven using UV-2101PC UV-Vis Spectrophotometer onscanning mode from 800 to 200 nm.

3. Results

Average Vickers hardness, standard deviations and con-verted hardness values are shown in Table 1A (2 h curing),Table 1B (24 h curing). Adhesion results are shown inTable 1A.

Abrasion resistance is often be characterized by using atransparent substrate such as polycarbonate and abradingthe surface with a specified load and number of cycles.Light transmittance as a function of wear cycles can thenbe compared between the coated and uncoated substrates.Fig. 1 shows percent transmittance in the wear track as afunction of wear cycles for uncoated polycarbonate andthe three coatings obtained after 24 h curing at 90 �C.

nd (C) GPCeO2 after 12 h UV curing.

0

100

200

300

400

500

600

10 20 30 40 50 60 70 80

Two-theta (deg)

Rel

ativ

e In

ten

sity

(A)

(B)

(C)

Fig. 6. XRD patterns of (A) GPSiO2, (B) GPCeZr and (C) GPCeO2 after12 h curing at 120 �C. XRD pattern of GPSiO2 confirms amorphousstructure, patterns of GPCeZr and GPCeO2 are in close accord with XRDpatterns of CeO2 and ZrO2.

Page 5: Particle-reinforced water-based organic–inorganic nanocomposite coatings for tailored applications

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

800750700650600550500450400350300250200

Wavelength (nm)

Ab

sorb

ance

A

B

C

Fig. 8. Absorption spectroscopy of coating sols diluted 10,000 times inH2O. (A) GPSiO2, (B) GPCeZr, (C) GPCeO2.

B.P. Mosher et al. / Journal of Non-Crystalline Solids 352 (2006) 3295–3301 3299

Fig. 2 shows SEM micrographs of the wear track area afterabrasion.

Fourier Transform Infrared Spectroscopy (FTIR) wasperformed before and after curing to investigate film struc-tures and verify the formation of the organic/inorganic net-work. FTIR spectra of GPSiO2, GPCeZr and GPCeO2

films are shown in Figs. 3–5, respectively. XRD patternsof cured films are shown in Fig. 6. TEM images of filmsare shown in Fig. 7. In order to determine the UV-absor-bance of the coating sols, each sol was diluted 10,000 timesin distilled water. UV-absorbance of the coatings is shownin Fig. 8.

4. Discussion

The coating sols contain FSO-100, a water-soluble eth-oxylated nonionic fluorosurfactant that gives exceptionallylow aqueous surface tensions. It also improves antisoiling,antifogging and UV resistance in coatings (Zonyl, DupontPerformance Chem.). All coatings also contain 3-glyci-doxypropltrimethoxysilane (GPTMS). GPTMS containsan epoxide ring that can be opened to form polyethyleneoxide chains, which enhance the mechanical propertiessuch as elasticity and lowers the thermal densification tem-perature of the films. We employ aluminum perchlorate asa catalyst, which has been previously demonstrated as ahighly effective catalyst for GPTMS organic polymeriza-tion [14]. The acid catalyzes the epoxy ring while at thesame time initiates inorganic hydrolysis and subsequentcondensation. Further curing locks in the nanocompositeSi–O–Si/C–O–C network.

GPSiO2 contains Ludox� TMA aqueous colloidal silica,composed of mono-disperse silica particles (�20 nm). Dur-ing drying, hydroxyl groups on the surface of the particlescondense by splitting out water to form siloxane bonds.The particles also develop strong adhesive and cohesivebonds and serve as effective binders in coatings [GraceDavison]. GPSiO2 also contains BYK-333, a silicone

surface additive which helps provide good surface wettingand anti-crater performance by reduction of surface ten-sion. BYK-333 is a polyether modified dimethylpolysilox-ane copolymer, containing no solvent (BYK-Chemie).There is no chemical interaction between GPTMS andBYK; the surface additive only provides enhanced physicalproperties. GPSiO2 synthesis is a two-part reaction. Acidcatalyst is added and the solution is allowed to stir whilehydrolysis and epoxy ring opening initiate. After adequatetime, surfactant and solvents are added to the solution toform a stable, pre-reacted sol which is ready for coatingand has a shelf life of up to one year. Unlike the othertwo coatings, in GPSiO2 silica nanoparticles chemicallylink with the network through Si–O–Si and Si–O–C bonds.

GPCeZr and GPCeO2 are very similar, differing only inconcentration and incorporation of metal oxide nanoparti-cles. GPCeZr contains cerium oxide and zirconium oxidenanoparticles; GPCeO2 contains only cerium oxide colloidand is more concentrated than GPCeZr (lower water con-tent). These solutions are synthesized through ‘one-pot’synthesis in which all the chemicals are combined simulta-neously and a stable homogeneous sol is formed in only10 min. The metal oxide nanoparticles are distributedthroughout the network in coatings prepared from thesesols.

The adhesion was very good for all coatings, especiallyGPCeZr and GPCeO2. For GPSiO2, the adhesion wasrated class 4B, indicating that small flakes of the coatingare detached at intersection with less than 5% of lattice pat-terns. For both GPCeZr and GPCeO2 the adhesion wasrated 5B, indicating 0% removal of the coating. The hard-ness of GPTMS based coatings increases with increase inthermal curing time [15]; comparing Tables 1A and 1Bshows a clear increase in the microhardness of all coatings(2 h vs 24 h curing at 120 �C).

From Fig. 1, it is clear that all coatings show very highabrasion resistance relative to polycarbonate with GPSiO2

films having the highest abrasion resistance. AlthoughGPSiO2 has the lowest hardness value, it is apparent thatBYK-333 additive and Ludox� TMA significantly increaseabrasion resistance. GPCeZr and GPCeO2 show very highabrasion resistance as well and even after 200 cycles under250 g load, they still show approximately 80% light trans-mittance in the wear track. Fig. 2 shows SEM images ofthe coatings after 250 wear cycles under 250 g load. A veryrough surface after abrasion indicates poor abrasion resis-tance; a smooth surface indicates high abrasion resistance.The images show that polycarbonate is very rough afterabrasion, whereas the coatings are still fairly smooth andthe wear mechanism can be clearly observed. This confirmsthe undoubted abrasion resistance improvements relativeto polycarbonate.

Figs. 3–5 show FTIR specta of GPSiO2, GPCeZr andCPCeO2, respectively, before and after curing. Glycidylethers show characteristic IR absorption bands at 1251–1241 (ring breathing), 912–920 (asymmetric ring deforma-tion), and 840–853 (symmetric ring deformation) cm�1.

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3300 B.P. Mosher et al. / Journal of Non-Crystalline Solids 352 (2006) 3295–3301

The C–H stretch in the epoxy also gives rise to anotherabsorption band at 3065–3038 cm�1 [16]. IR spectrum ofGPTMS shows epoxy bands peaking at 3050, 1254 and855 cm�1, which were used as a reference to monitor epoxyring opening by Al(ClO4)3.

The peaks around 1220 and 1090 cm�1 are associatedwith Si–O–Si/C–O–C asymmetric bond stretching vibrationwere present in all samples. Peaks around 750–800 cm�1

associated with Si–O–Si symmetric bond vibration werealso observed in all samples. The broad bands centerednear 3400 cm�1 are assigned to residual Si–OH stretchingvibrations and hydrogen-bonded water [17]. Signals at910 cm�1 arise due to both epoxy groups and Si–OH orSi–O� groups [18]. All dried samples (Figs. 3(A), 4(A),5(A)) show weak residual epoxy signals at 855, 1250 and3050 cm�1.

After 2 h curing at 120 �C (Fig. 3(B)), GPSiO2 shows noresidual epoxy groups and a highly crosslinked organic/inorganic network has formed as shown by the strongbroad signal �1090 cm�1. GPCeZr and GPCeO2 showvery weak epoxy bands at 855 cm�1 after curing, but showno epoxy bands at 1250 cm�1 or 3050 cm�1, indicatingmost epoxies have reacted. GPSiO2 shows completeorganic polymerization after 2 h curing due to slightlymore catalyst and incorporation of colloidal silica, whichwe have shown to expedite GPTMS crosslinking. Nano-particles in all the materials serve as nucleation sites to pro-mote polymer precipitation, allowing for very little catalystuse. However, silica particles demonstrate a secondary cat-alytic effect due to surface silanols, which contribute toepoxy ring opening.

The absorption band around 820 cm�1 in GPTMS, whichis due to Si–O (CH3) symmetric stretching vibration [19,20],has vanished in all samples indicated inorganic hydrolysishas occurred. Hydrolysis is further confirmed by the changein the GTPMS absorption bands at 2940 and 2840 cm�1

which are attributed to C–H asymmetric and symmetricstretching in Si–O–CH3, respectively. As with GPSiO2

GPCeZr and GPCeO2 show formation of organic/inorganicnetworks by strong broad Si–O–Si/C–O–C bands around1090 cm�1 and 1200 cm�1. Si–O–Si stretching vibrations inlinear and less cross-linked structures give rise to bandsaround 1120 and 1160 cm�1 [21]. These shoulders are notobserved in any of the films, indicated highly cross-linkedcyclic structures. Also, the 1200 cm�1 is overlapped by theSi–CH2 wagging vibration band [22].

Fig. 6 shows XRD patterns of coatings obtained after12 h curing at 120 �C. All films are amorphous; the pat-terns for GPCeZr and GPCeO2 are in good accord withthose of pure ZrO2 and CeO2. The TEM micrograph inof GPSiO2 in Fig. 7(A) clearly show a worm-like amor-phous network with silica particles homogeneously dis-persed throughout. Non-ionic (NI0) surfactant pathwayslead to amorphous worm-like structures as seen in the fig-ure. Figs. 7(B) and (C) show images of GPCeZr andGPCeO2, respectively. The particle reinforcement in thesematerials is not mono-disperse as in GPSiO2 so resulting

films have some particle agglomeration during synthesisand drying. Regardless, the amorphous, porous, worm-likestructure results and mechanical and optical performanceremains.

Fig. 8 shows UV absorption of the coatings. From thefigure, GPSiO2 shows little UV absorbance, although thereis a small amount of UV-absorbance on the lower end ofthe spectrum (around 200 nm) which is likely dueorganic/inorganic network absorption. However GPCeZrand GPCeO2 show strong absorbance of UV waves, espe-cially from 200 to 300 nm, which are very harmful tohumans. This UV absorption is due to the incorporationof metal oxide nanoparticles. GPCeO2 shows much stron-ger absorption due to its higher concentrations (GPCeZris diluted 5:3 with H2O). When large particles are used asadditives in polymers, scattered UV light concentrates nearthe particles, which can cause photodegradation in theregion around the particles. The intensity of the light scat-tered from a particle is governed by the Mie Equation, andtherefore as the particle size approaches the nanoscale, theparticles effectively absorb UV radiation rather than scatterit [23]. The particles employed in this experiment are 5–20 nm in diameter, therefore are extremely effective inabsorbing UV light. The metal oxide nanoparticles alsoabsorb 300–400 nm UV irradiation, which can reach theearth’s surface and cause skin disease. Thus GPCeZr andGPCeO2 have important applications in protecting humansor other material sensitive to UV irradiation [24].

5. Conclusions

We have synthesized three organic–inorganic water-based nanocomposite coatings, which are relatively hardand abrasion resistant with very good adhesion. Two ofthe coatings are useful for protecting humans or otherUV sensitive materials from harmful UV irradiation.Incorporation of nanoparticle colloids allows minimal cat-alyst use and provides facile and green synthesis of coatingsols which have long shelf life. Short curing at low temper-ature concludes polymerization to produce dense, hardfilms. All coatings have unique properties and can servenumerous applications, especially as protective and multi-functional coatings. The synthesis and application of allcoatings is simple, fast and produces very few harmfulbyproducts, making them ideal materials in industry.

Acknowledgements

This project is partially supported by grants from NSF(CTS-0500402) and DoE (DE-FG02-05ER46241) througha subcontract of MIT.

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