Study And Characterization of Organically Modified Silica-Zirconia Anti-Graffiti Coatings Obtained...

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J. Chem. Chem. Eng. 7 (2013) 120-131

Study and Characterization of Organically Modified

Silica-Zirconia Anti-Graffit i Coatings Obtained bySol-Gel

M. Rosario Elvira, M. Alejandra Mazo *, Aitana Tamayo, Fausto Rubio, Juan Rubio and Jos Luis OteoCeramic and Glass Institute, CSIC, Madrid 28049, Spain

Received: November 28, 2012 / Accepted: December 21, 2012 / Published: February 25, 2013.

Abstract: In this work, it is presented the synthesis and characterization of transparent and colorless organic-inorganic hybridanti-graffiti protective materials obtained by sol-gel method. This type of materials is based on MTES (methyltriethoxysilane), TPOZ(tetrapropoxide of zirconium) and PDMS (polydimethylsiloxane). The synthesis has been carried out at 25, 35 and 45 C in order toevaluate the role of temperature in the structure, microstructure and anti-graffiti behavior as well. The incorporation of zirconiumwithin the organic modified silica network, of sols after being gelled and dried, is evident by a shoulder which increased withtemperature situated at 950 cm -1 (Si-O-Zr bonds), and it is homogenously dispersed inside the matrix avoiding the formation of largeZrO 2 precipitates. As the temperature increases, the hydrolysis and condensation reactions occur in more extension and thus, theobtained sols are more cross-linked and present more Si-O-Zr linkages. The promising anti-graffiti behavior of the protective hybridswas qualitatively determined being the spot removal higher than 90%.

Key words: Sol-gel, organic-inorganic hybrid, anti-graffiti coatings, hydrophobic materials.

1. Introduction

Anti-Graffiti materials have attracted much attention

during the last decades because graffiti paintings are

present in the routine life, especially in big cities.

Graffiti can affect to all class of surfaces materials and

in the majority of the cases, the cleaning is very

expensive and quite often, the penetration into the

pores contained in the substrate material induces an

irreparable effect onto the painted surface. The first

attempts to remove the graffiti, basically included theremoval of the painting by employing pressurized

water and brush, but these methods are not so effective

and require a large amount of water and labor, with the

risk of damaging the painted surface during the

cleaning process. Based on these facts, it came out the

*Corresponding author: M. Alejandra Mazo, Ph.D., researchfields: materials science (organic-inorganic hybrids, siliconoxycarbide glasses, composites, etc.). E-mail:[email protected].

necessity of developing new products that could beused as permanent protective coatings easily applied

over the surfaces sensitive of being damaged. The most

recent efforts consist on the application of a product

which makes the surface highly hydrophobic

independently on its nature and protects it against water,

minimizes the adhesion of the graffiti paint and

facilitates the removing process with water, soft

detergents [1] or hand cleaning by employing a cloth.

Organic-inorganic hybrids based on methyl-derived

silanes present highly hydrophobic properties and high

specific surface energy avoiding the adherence of the

graffiti paint [2, 3]. The combination of these

properties gives as a result a new family of materials

susceptible of being used for anti-graffiti purposes

making the subsequent removal lighter [2]. Other

family of anti-graffiti coatings developed in the last

few years also include the use of polysiloxane modified

polyurethanes whose weathering resistance is

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Study and Characterization of Organically Modified Silica-Zirconia Anti -Graffi ti Coatin gs Obtain ed b y Sol -Gel

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increased by adding silica nanoparticles [4]. The

development of fluorinated polyurethane dispersions

as anti-graffiti materials provides even

super-hydrophobic surfaces but their main drawback is

that these solutions possess as non-environmentally

friendly compounds because of the presence of fluorine

in their compositions [5, 6]. The enhanced chemical

and mechanical properties of the ZrO 2-SiO 2

combinations compared with silica or zirconia leaded

to the appearance of different compounds susceptible

of being used for this purpose containing a mixture of

both oxides [7, 8]. In this sense, organic-inorganic

hybrids with excellent homogeneity, mechanicalintegrity and transparency have been synthetized by

employing PDMS (polydimethylsiloxane), as

precursor of organically modified silica, and zirconium

TPOZ solutions [9, 10]. The combination of both

solutions into a single material and the addition of

MTES, produced a sol-gel organic-inorganic hybrid

highly transparent and colorless coating [11]. When

they are applied over a dense material (granite and

marble) the color of the substrate remains unchangedand also presents good anti-graffiti properties which

also remains unaltered against weathering conditions

(rain and UV radiation) and made them suitable as a

protective coating for building facades. When the

material presents some amount of porosity (i.e.,

sandstone) the coating penetrates inside the pores

inducing significant changes in the material color while

the anti-graffiti effect is reduced.

Several authors studied the effectiveness of

MTES/PDMS/TPOZ organic-inorganic hybrid

anti-graffiti protective coatings by measuring the

degree of penetration in the materials, durability

against pollution and the determination of the surface

dispersive energy after being deposited over different

building materials (cement paste, lime mortar,

limestone, granite and brick) [12-14]. Numerous

factors are decisive in order to explain the anti-graffiti

behavior of protective coatings some of them are: the

wettability, the morphology, the roughness, the surface

energy and the chemical composition of surface

coating [15]. Anti-graffiti protective coatings reduce

the surface energy of the substrates which hinders the

interaction with the graffiti paint and makes the

removal easier [14].

With all these considerations, the aim of this work is

to develop an anti-graffiti protective organic-inorganic

hybrid coating by employing silicon and zirconium

derivates MTES, TPOZ and PDMS and the study of the

influence of synthesis conditions on the structure,

microstructure and the final anti-graffiti behavior.

2. Experiments

2.1 Reagents

The sol-gel procedure was used to obtain

MTES/TPOZ/PDMS organic-inorganic hybrid

coatings. The reagents used in the synthesis are methyl

triethoxysilane (MTES, 98% purity, ABCR GmbH &

Co.), zirconium (IV) propoxide solution (TPOZ, 70%

wt. in n-propanol, Sigma Aldrich Co.), and hydroxyl

terminated polydimethylsiloxane (PDMS, 4%-6% OH

content, ABCR GmbH & Co.). The solvent used wasn-propanol (n-PrOH, Sigma Aldrich Co.).

2.2 Synthesis

The synthesis was carried out by a method

previously described by Oteo, Rubio and Rubio [16].

This procedure yields a homogeneous

organic-inorganic network from an organic modified

silicate (MTES), an organic derived siloxane polymer

(PDMS) and a zirconium second inorganic network

formed by employing a zirconium tetrapropoxide as

precursor (TPOZ).

The weight ratio employed of MTES/PDMS/TPOZ

is 50/40/10 and the molar ratio of MTES/n-PrOH is 1/5.

First, MTES, PDMS and the half volume of n-PrOH

are mixed during approximately 10 min to ensure the

homogeneous mixing of the precursors. Separately, the

other half of n-PrOH and the TPOZ are also mixed.

Then, the mixtures were added into a flask under reflux,

magnetic stirring and thermo-stated, varying with a


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temperature controller in order to study the influence of

this parameter in the organic-inorganic hybrid from 25

to 45 C. According with preceding studies, the

reaction time was set to 22 h [11]. After that,

transparent and homogeneous sols are obtained and

when ambient temperature is reached they can be

transferred into hermetically sealed containers. Sols

can be maintained in these air and humidity-free

containers until use, avoiding further hydrolysis,

gelling and solvent evaporation processes during

several months.

Hybrids synthetized at different temperatures (25, 35

and 45 C, named as R25, R35 and R45, respectively)were poured within a PS (polystyrene) petri dish and let

for 5 days until gelling, and then they were dried at 50

C for another 6 days. After that, one part was reduced

to powder and used for the full characterization of the

material and other part was used to test the anti-graffiti

behavior.

The anti-graffiti behavior is analyzed by using spray

painting and a permanent marker. The spray (PINTY

PLUS Basic NOVASOL SPRAY S.A.) application

was carried out at a distance of 10 cm distance from the

gel surface placed perpendicularly to the sprayed

substance. It was used a circular template to produce

sprayed spots as similar as possible for comparison of

the effectiveness of the cleaning. After the paint is

dried, the sprayed spots were tried to be clean with a

cotton tissue. The second experiment for testing the

anti-graffiti properties was performed with a

permanent marker Edding 3000 by writing down a

line over the organic-inorganic coating surface. Oncethe ink was dried, the marker spot was tried to be clean

with a cotton tissue. In both experiments, a blank

surface (the original Petri Dish) with no treatment

applied was used to compare the results obtained.

2.3 Material Characterization

Organic-inorganic samples were analyzed by means

of infrared spectroscopy. Sol samples after 22 h of

sol-gel reaction were studied by infrared spectroscopy

in the transmission mode from 4,000-600 cm -1 by a

FT-IR (fourier transform infrared) spectrometer. The

measurements were carried out by adding one drop of

the sol between two KRS-5 windows, which are

transparent to the infrared radiation. Powdered samples

were analyzed by ATR (attenuated total reflection)

mode in the spectral range 4,000-600 cm -1 using the

MIRacleTM attenuated total reflectance device. In

both cases, it were used a perkin elmer spectrometer

model Spectrum BX. Eight scans were used to obtain

each spectrum and the background was subtracted from

all spectra. The resolution was 4 cm -1 in the spectral

range.Also, ATR was made in the Far-IR spectral range

(600-200 cm -1) with a Thermo Scienti c Nicolet

6700 FT-IR spectrometer. The spectrometer was

equipped with the Thermo Scienti c ETC EverGlo*

ceramic IR source and Far-IR optics (solid substrate

beamsplitter and polyethylene DTGS detector). 512

scans were collected at 4 cm -1 resolution. Powdered

samples were also analyzed by raman spectroscopy;

the spectra were carried out in a spectrometer of

renishaw model In via with an excitation wavelength of

514 nm corresponding to Ar + laser. 29Si MAS NMR

spectra of organic-inorganic hybrids were collected

with an AV-400-WB Bruker Corp, NMR spectrometer

(79.49 MHz). The spinning rate for samples was 10

KHz, the spectra were recorded using a pulse /6 (2 s)

at 50 kHz and delaying time of 60 s. The observed

silicon units were designed according to the usual

notation employed in silicon chemistry: Q [SiO 4], T

[SiCO 3], D [SiC 2O2], M [SiC 3O] and X [SiC 4]. Thenumber added to the unit symbol as a subscript

indicates the number of oxo bridges bonds to the

corresponding Si site.

TG (thermogravimetric) studies and DTA

(differential thermal analysis) experiments were

carried out in a TA Instruments SDT Q600 with

simultaneous TGA/DTA analysis. The analysis

conditions were heating rate of 5 C/min under flowing

air (100 mL/min).


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The total carbon and hydrogen contents were

performed by using elemental analyzers, LECO model

CS-200 and RC-412, respectively. Silicon and

zirconium content was determined by X-ray

fluorescence, Philips model Magix. Oxygen content

was determined by difference.

The microstructure of the protective films was

observed by field emission scanning electron

microscopy (FE-SEM, Hitachi 4700) working at 20 kV.

In selected areas, mapping experiments were also

performed for silicon and zirconium elements.

3. Results and Discussion

3.1 IR, Raman and 29Si-MAS NMR

Fig. 1 shows the FT-IR spectra of the sols obtained at

different synthesis temperature in the spectral range

1,350-600 cm -1. As reported by Rubio et al. [17], the

band appearing at 1,153 cm -1 and corresponding to the

propoxy groups of TPOZ is the most suitable in order

to follow the hydrolysis of this compound. In the

spectra, it is clearly observed that as the temperature

increases the intensity of the referred band drasticallydecreases from R25 to R45, as consequence of the

major extension of the hydrolysis reactions. The

decrease of the intensity of this band occurs together

with the increase of the bands related to n-PrOH

because of the formation of new molecules of alcohol.

Moreover, the bands centered at 1,170 and 960 cm -1

(Si-O-C asymmetric and symmetric stretch, respectively)

Fig. 1 FT-IR spectra of the sols obtained at differentsynthesis temperature after 22 h.

[18] that correspond to the ethoxy groups of MTES

permit the evaluation of the degree of hydrolysis and

condensation of MTES molecules. As observed in the

analysis of the TPOZ bands, MTES bands, especially

the one located at 1,170 cm -1 are getting smaller and

almost disappear as the synthesis temperature increases,

but the bands of pure EtOH increases in intensity from

R25 to R45 (seen 1,230 cm -1 band in Fig. 1).

The overlapping of multiple vibrations is

represented as a broad band in the 940-980 cm -1

spectral range which difficult the evaluation of the

contribution of each one to the total intensity of the

band. These vibrations correspond to the stretchingmode of Si-OH groups (~ 950 cm -1), the ethoxy groups

of MTES molecule (~ 960 cm -1), the n-PrOH molecule

(~ 969 cm -1) and Si-O-Zr new linkages (~ 980 cm -1)

[19]. A careful analysis of the spectra indicates that the

950 cm -1 (Si-OH) and 960 cm -1 (Si-OCH 2CH 3) bands

decrease with temperature but the 969 cm -1 band

(n-PrOH) becomes stronger. These facts imply a major

extension of the hydrolysis and condensation reactions

of MTES and TPOZ molecules, as commented before.

On the other hand, the band centered around 980-920

cm -1 exhibits a shift to lower wavenumbers with the

synthesis temperature. The shift is attributed to the

formation of new linkages Si-O-Zr, as occurs in the

case of PDMS/Metal alcoxide hybrids in which is

reported that the Si-O-M band shifts to 910-930 cm -1

because of the presence of the mixed bonds [19].

IR-ATR spectra of the hybrid materials obtained

after gelling and drying are presented in Fig. 2a and 2b

for MIR and FIR spectral range, respectively. The bands related to the methyl groups of the precursors

MTES and PDMS appear all over the spectra whose

main characteristics are the absorption bands centered

at 2,966, 2,908 cm -1 attributed to the asymmetric and

symmetric C-H stretching vibrations. The symmetric

bending mode of CH 3 groups appears as a very intense

band placed at 1,260 cm -1 which it is related to

D (SiMe 2) and T (SiMe) units present in PDMS and


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Fig. 2 ATR spectra of the hybrids obtained at differenttemperatures heat-treated at 50 C for 6 days: (a) MIR and (b)FIR.

MTES precursors, respectively [20]. The band placed

at 857 cm-1

is related to symmetric rocking of CH 3,whereas band centered at 700 cm -1 is associated to the

symmetric stretching of Si-C bonds. The asymmetric

stretching of Si-C bonds appears as a very broad band

centered at wave numbers below 800 cm -1. In the case

of PDMS polymers, the position of this band is related

to linear (800 cm -1) or cyclic (810 cm -1) polymer chains

conformation [21]. The shift of the band to a lower

wave numbers suggests that the polymer chains of the

organic-inorganic hybrid could adopt a linear

conformation. This band is slightly overlapped with the

symmetric stretching of Si-O-Si bonds at 756 cm -1.

Other bands related to Si-O bonds coming from

MTES and PDMS are located approximately at 1,080

and 1,010 cm -1 and are assigned to asymmetric

stretching of Si-O bonds in the LO (longitudinal) and

TO (transversal) active modes, respectively [22]. In the

far IR range, the band placed at 370 cm -1 and

corresponding to bending mode of O-Si-O bonds is

shifted with respect of the same vibration in pure silicagels [23]. This downshift is reported in modified silica

network with polymers or other type of oxides (i.e.,

metallic oxides or ZrO 2, TiO 2, Al 2O3, etc.) because of

the continuous interruption of the tridimensional

Si-O-Si network with Si-CH 3 or Metal-O linkages [3, 9,

24]. The shift of the TO mode of the Si-O asymmetric

stretching to lower wave number (approximately from

1,100 to 1,040 cm -1) has been also reported in

Zr-modified silica because of the formation of Si-O-Zr

hetero-linkages being the shift directly related to the

ZrO 2 content [25-27].

The broad shoulder centered at ~ 950 cm -1 can

include both Si-OH groups (Si-OH stretching mode)

and to Si-O-Zr bonds. The absence of the OH vibration

bands at around 3,500 cm -1, suggests that the shoulder

could indicate the presence of Si-O-Zr linkages [28].

The broadness of the shoulder is attributed to the

presence of Si-O-M bonds with Si coming either form

MTES or PDMS, in which the band shifts to 910-930

cm -1 [10]. It is observed that as the synthesis

temperature increases, the 950 cm -1 shoulder is getting

bigger and well resolved which implies a higherSi-O-Zr concentration.

The bands placed at 650 cm -1 (Zr-OX, X = H,

CH 2CH 2CH 3) and 500-460 cm-1 (Zr-O-Zr bonds)

appearing in pure zirconia based gels [7, 9, 29] are

practically imperceptible in the FT-ATR spectra. This

fact implies that TPOZ is almost completely

hydrolyzed and zirconium oxo networks are mixed

with PDMS and MTES building a mixed organic

modified silica-zirconium oxide network and ZrO 2 is

not segregated. The steric hindrance of PDMS and the

copolymerization through end chain silanol groups

with metal alkoxide prevents the growth of ZrO 2 particles during the hydrolysis and condensation

reactions [10]. ZrO 2 small clusters are mixed

homogeneously with PDMS and MTES molecules and

the zirconium oxo networks (Zr-O-Zr stretching

vibrations) only appears as a very small band located

approximately at 500 cm -1.

Fig. 3 contains the Raman spectra of theorganic-inorganic hybrid materials. Similarly to the

infrared analysis, the bands related to the organic

groups in either the PDMS or MTES molecules

contained in the hybrid material, are the most

characteristic bands in the spectra. These bands are

assigned to the asymmetric and symmetric CH

stretching vibrations (2,968 and 2,908 cm -1) and

asymmetric and symmetric bending mode to CH 3

groups of both D and T groups (1,416 and 1,261 cm -1,


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Fig. 3 Raman spectra of organic-inorganic hybrids obtainedat different synthesis temperatures heat-treated at 50 C for 6days.

respectively). Another bands related to organic methyl

groups are placed at 860, 707 and 683 cm -1 which are

related to symmetric rocking of CH 3 groups,

symmetric stretching of Si-C bonds and symmetric

rocking of Si-CH 3 bonds [30]. Asymmetric LO and

TO stretching modes of the Si-O bonds (1,200 to

1,000 cm -1) yield a low intensity Raman band while

its symmetric modes are very intense (790 and 760

cm -1 , respectively). The asymmetric stretching mode

of four members rings of the modified silica network

the siloxane units in the middle of PDMS chains

appears as an intense band at 490 cm -1 [28]. Galleener

et al. [31] in their studies with quartz associated the

formation of these four member rings with the

presence of defects in the crystalline network. In the

organic-inorganic hybrid materials described in the present work, the presence of PDMS molecules and

ZrO 4- x could also be the responsible of the formation

of these four member rings [32, 33].

Finally, in the case of R45, a small band placed at ~

900 cm -1 is noticeable. This band could be assigned to

Si-O-Zr linkages as well as it was observed in ATR

spectrum and agreed with the supposition that as the

temperature increases the amount on Si-O-Zr linkages

are higher.

Fig. 4 29Si MAS NMR spectra of the organic-inorganichybrids synthetized at different temperatures heat-treated at50 C for 6 days.

29Si MAS RMN spectra of organic-inorganic hybrids

are presented in Fig. 4. In the spectra, there are clearly

distinguishable two types of signals related to the

different structures of PDMS and MTES, D (SiC 2O2)

and T (SiCO 3) units, respectively, which permitauthors to elucidate the silicon backbone of the

organic-inorganic hybrid material. Short PDMS chains

(n 5), which presents peaks placed below -22 ppm,

and long PDMS chains (n >> 10) are distinguishable in

terms of the chemical shift because the peak is

presented at higher chemical shift when the polymeric

chain increases [34]. In all the cases, a peak at

approximately 22.5 ppm (Table 1) corresponding to D 2

units of lineal PDMS chains [23] indicates that theorganic-inorganic hybrids are formed mainly by long

lineal PDMS chains. Several authors [28, 35] reported

in the 29Si MAS NMR cross polarization experiments

(29Si-CP MAS NMR) of PDMS/Metal oxides hybrids a

broadness of this signal at chemical shifts higher than

-22 ppm with respect to 29Si-MASNMR spectrum

without cross polarization and associated the intensity

of the peak to less mobile units, like for example cross

linking points with metal oxides or the end of the


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Table 1 Data extracted from deconvolution of 29Si MASNMR spectra of the organic-inorganic hybrids synthesized atdifferent temperatures (25, 35 and 45 C).

T (ppm/%)D (ppm/%)

R25 -67.730-66.1

12-22.4

52-19.9

6

R35 -66.724-66.1

11-22.5

57-19.9

8

R45 -67.722-66.2

12-22.4

57-19.9

9

PDMS chains. The sharpness of the mentioned peak

indicates the high mobility of PDMS chains and

explains the high flexibility of hybrids made them

suitable for being obtained as crack free coatings [35].

Besides the main D 2 peak, centered at -19.9 ppmappears a small peak which could be associated to

cyclic oligomers entrapped into hybrid network. The

intensity of this band rises from 6%, 8% and 9% as the

temperature increases from 25, 35 to 45 C,

respectively (Table 1).The T 3 peak corresponding to T 3

units derived from the MTES molecule is quite broad

but it is possible to differentiate between two peaks

close to -66.1 and to -67.7 ppm, assigned to Si-O bonds

with different chemical environment. These two peaks

are tentatively assigned to Si-O-Zr and Si-O-Si

linkages, respectively (Table 1).

3.2 Thermal and Chemical Analysis

With the aim of distinguishing the changes induced

into the hybrid network because of the synthesis atdifferent temperatures, thermogravimetric and

differential thermal analysis have been carried out. In

Fig. 5 (a, b) it is represented the TG/DTG (derivate of

thermogravimetric analysis) and DTA experiments of

the organic-inorganic hybrid synthetized at the three

analyzed temperatures. According with the

thermogramme, thermal degradation occurs mainly in

four steps showing a total weight loss of 31%, 32% and

27% for samples obtained at 25, 35 and 45 C. Beyond

600 C the thermal degradation is almost complete inall the materials and no significant changes were

observed. According to DTA curves, all the

phenomena occurring during the four decomposition

stages are exothermic. Table 2 collects the

temperatures and weight loss associated with the

referred temperature corresponding to the main peaks

in the DTA curve.

In the first step, from room temperature to ~ 430 C,

it is produced the evaporation of the entrapped solventwithin the coating and a further cross-linking processes

between remainder SiOR groups (R = H, CH 2CH 3)

Fig. 5 (a) TG/DTG and (b) DTA curves of the hybrids obtained at 25, 35 and 45 C.

Table 2 Main thermal data obtained by TG/DTG and DTA curves: temperature ranges, DTG peaks, weight losses (total weightloss (WLt) and related to each stage (WLs) measured in the organic-inorganic hybrid synthetized at 25, 35 and 45 C.

Firststage(C)

First peak(C)

WLt(WLs1)(%)

Secondstage(C)

Second peak(C)

WLt(WLs2)(%)

Thirdstage(C)

Third peak(C)

WLt(WLs3)(%)

Fourthstage(C)

Fourth peak(C)

WLt(WLs4)(%)

R25 r.t-435 359 12 (12) 435-462 450 15 (3) 462-500 485 23 (8) 500-551 516 30 (7)R35 r.t-417 373 13 (13) 417-441 423 15 (2) 441-481 459 20 (5) 481-559 510 28 (8)R45 r.t-416 371 13 (13) 416-439 429 15 (2) 439-478 464 18 (3) 478-557 507 26 (8)


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rendering new Si-O-Si linkages. Both the temperature

range and the total WLs1 (weight loss observed in this

first step) are quite similar for all the synthesistemperature analyzed with a weight loss of about 13%.

The broadness of the peak found in the DTA curve is

attributed to the simultaneous reactions that take place

in this temperature range.

The second step that is shown as a small peak in the

DTA curve, is attributed to the removal of volatile

cyclic oligomers (essentially three and four member

rings D 3c and D 4c, respectively) derived from PDMS. In

this second stage, the relative weight loss with respect

to the remainder after the first decomposition (WLs2)

decreases with synthesis temperature, from 3% (R25)

to 2% (R45) (Table 2), and suggest a more cross-linked

structure because of a larger extent of the hydrolysis

and condensation reactions during sol-gel process

when the synthesis temperature increases.

The main weigh losses in the thermogramme

correspond to the third and fourth stages, and are

associated with the depolymerization and oxidation of

the PDMS. As shown in Table 2, the contribution to thetotal weight loss of each decomposition step is deeply

influenced by the synthesis temperature being the

WLs3 of about 8% in the R25 sample and around 3% in

R45, whereas the WLs4 remains practically invariable.

These observations are in accordance with Camino et al.

[36], that reported that the Si-O/Si-O and Si-O/Si-C

exchange reactions in PDMS materials produce the

elimination of polymer chains through the formation of

volatile cyclic oligomers (mainly D 3c and D 4c) together

with the oxidation of the methyl groups bonded to thesilica network. At the same time, the temperature also

promotes the cross-linking of the structure and

enhances its thermal stability, which is translated in a

delay in the oxidative thermal degradation of the

organic part still present into the thermally degraded

hybrid network. The thermal oxidative degradationtakes place in a temperature range that increases with

the synthesis temperature increases which denote a

more cross-linked structure [37]. The lower WLs3 in

the hybrid obtained at 45 C compared with the one

synthesized at 25 C is explained in terms of the higher

condensation degree of the structure in the former one.

Table 3 shows the results obtained from the chemical

analysis of the obtained organic-inorganic hybrids. It is

found that the empirical formula of the synthesized

products is slightly dependent on the synthesis

temperature. The ratio C/H equal to 3 in all the cases

indicates that the hydrolysis is almost complete and

comparing with the theoretical formula, it is observed

that O/Si molar ratio slightly decrease with the

synthesis temperature. This result suggests the

presence of non-condensated Si-OH moieties,

especially in the material synthesized at the lowest

temperature.

3.3 SEM

Fig. 6 presents the microstructural and

compositional SEM analysis performed in R25 and

R45 samples. The images of the surface (left images)

show almost fully dense hybrid materials. This fact is

very important because the presence of pores may

reduce the effectiveness of the anti-graffiti protective

coating. These images display nodes and groove marks

associated to the shrinkage experimented during

gelling and subsequent drying which are more evidentin the material synthetized at 25 C than the one

obtained at 45 C. This surface morphology is

directly related with the roughness of gel, so the mayor

Table 3 Analytical results, theoretical and experimental formula of the organic-inorganic hybrid materials.

Sample Si (%) O (%) C (%) Zr (%) H (%) Empirical formulaTheoretical 36.04 27.63 25.50 4.46 6.37 SiO 1.25C1.65H4.95Zr 0.04 R25 33.70 29.68 25.60 4.63 6.39 SiO 1.54C1.77H .31Zr 0.04 R35 33.65 28.05 27.25 4.24 6.81 SiO 1.46C1.89H5.67Zr 0.04 R45 33.35 28.52 26.88 4.58 6.67 SiO 1.50C1.88H5.60Zr 0.04


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roughness in R25 may reduce its durability. During sol

to gel transformation and also in drying stage, the

condensation reactions produce a shrinkage which produces the above mentioned nodes and grooves, in

the case of R25 more unreacted groups are susceptible

to react and as a result the total amount of shrinkage is

greater. This assumption agrees perfectly with above

mentioned results found in TG/DTG analysis, chemical

analysis and FT-IR spectra.

SEM-mapping of silicon and zirconium was

performed and the images are also displayed in Fig. 6

(center and right images, respectively). Mapping shows

that silicon and zirconium are well and uniformly

dispersed within the coating. As expected, the silicon

amount is higher than zirconium according to the

chemical composition (Table 3), but in any case large

aggregates of zirconium are observed.

3.4 Anti-graffiti Behavior

The anti-graffiti behavior was tested by using

different types of ink, a spray painting and a permanent

marker as shown in Figs. 7 and 8, respectively.Fig. 7 shows the results obtained when a spray is

applied over the protective gel synthetized at different

temperatures. A substrate without any coating is used

as blank surface (PS Petri dish). As it can be seen,

except in the case of the blank substrate, the behavior

of the protective gel after being sprayed and cleaned is

almost similar independently of the synthesis

temperature. As a qualitative approximation of the total

amount of ink-spot removal, it is estimated that in all

the cases the elimination is higher than 90%. However,in the case of blank substrate, the ink-spot cannot be

removed. Due to the hydrophobic nature of the

protective gel, after the application of the spray the ink

is not well adhered over the protective coating and this

fact allows the easy removal of the majority of the spot

produced by the spray. However the removal is not

totally complete and an exterior circular halo of ink is

still present after the cleaning procedure regardless of

the synthesis temperature. The cleaning efficiency was

also evaluated according to different CN (classification

numbers) [38, 39], which varied from CN 0 (total

removal) to CN 5 (no cleaning effect). Theclassification number displayed for all the protective

coatings are assigned to CN 1.5 which means that the

spot persists as a single laminar residue of color.

An analogous experiment was carried out by

employing a permanent marker, as shown in Fig. 8. The

Fig. 6 SEM characterization of organic-inorganic hybrids.Left images correspond with surface of coatings synthetizedat 25 C (a) and 45 C (b), center and right images areassociated to SEM-mapping analysis of silicon and zirconium,respectively.

Fig. 7 Anti-graffiti behavior against spray painting of thehybrids obtained at different temperatures (45, 35 and 25 C).

Fig. 8 Anti-graffiti behavior against permanent marker of

the hybrids obtained at different temperatures (45, 35 and

25 C).


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129

ink spot is easily and totally removed (100%) in all the

cases except in the blank substrate. The cleaning

efficiency employing the classification numbers [38,

39] is CN 0 assigned to a total removal of the paint.

The anti-graffiti behavior could be related both to the

gel hydrophobic character due to the hydrophobic

nature of the material attributed to the presence of

organic groups (methyl groups) and to the densification

as well to the homogeneous and non-porous surface

reached after drying. Therefore, these gels may act like

a protective barriers and do not permit the spot

penetration inside the coating and allow the easy

removal of the paint.

4. Conclusions

A catalyst free sol-gel procedure is used to

synthetize an organic-inorganic hybrid solution mainly

formed by organic modified silica and zirconium,

which can be used as anti-graffiti protective gel. As the

synthesis temperature is increased from 25 to 45 C, the

hydrolysis and copolymerization reactions occur in a

larger extent and as a result, the resulting sol is more

cross-linked and the presence of Si-O-Zr new linkages

is more obvious. During sol to gel transformation and

drying step the protective materials formed present in

all the cases Si-O-Zr new linkages detected by a broad

shoulder centered at ~ 950 cm -1 in IR-ATR

spectroscopy. A shift in both Si-O stretching and

bending bands is also observed and is due to

incorporation of both polymer and zirconium within

the silica network. Based on the structural analysis

performed, the organic-inorganic hybrid material isformed by long lineal chains of PDMS polymer

condensed with both TPOZ and MTES derived

molecules, additional D 4 cyclic oligomers are

entrapped inside this structure with increasing amounts

with the synthesis temperature.

The organic-inorganic gel surface also displays a

nonporous and homogeneous final appearance.

SEM-mapping analysis shows zirconium is

homogeneously dispersed within the

organically-modified silica matrix, and large zirconia

precipitates are not detected. However as the

temperature increases, when sol to gel transformation

is produced, the final protective material synthetized at

45 C undergoes lower shrinkage which minimizes the

apparition of voids and grooves and as a result the

surface is smoother which imply more durability. The

anti-graffiti response of the protective coating is very

promising for spray and marker paint, and the spot

removal determined qualitatively is > 90 and of 100%,

respectively.

Moreover, these products can be used as protective

coatings of a great variety of materials, like forexample glass, metals, alloys, stones (marble, granite,

sandstone), concrete, etc.. Other experiments are now

under consideration in order to test other properties

useful for different kind of applications, i.e., color,

gloss, water absorption, durability, etc..

Acknowledgments

The authors thank the Spanish Ministry of Economy

and Competitiveness for the financial support of the

project Sntesis de nanopartculas multifuncionales deoxicarburo de silicio para aplicaciones biomdicas,

reference number MAT2009-14450.

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