Fabrication of super-hydrophobic surfaces for enhanced stone protection

1. Fabrication of super-hydrophobic surfaces for enhanced stone protection
P.N. Manoudis a
, A. Tsakalof b,
⁎, I. Karapanagiotis c
, I. Zuburtikudis d
, C. Panayiotou a
a
Aristotle University of Thessaloniki, Department of Chemical Engineering, 54124, Thessaloniki, Greece
b
University of Thessaly, Medical Department, Larissa, 41222, Greece
c
Ormylia Foundation, Art Diagnosis Centre, Ormylia, Chalkidiki, 63071, Greece
d
TEI of Western Macedonia, Department of Industrial Design Engineering, Kozani, 50100, Greece
a b s t r a c t
a r t i c l e i n f o
Article history:
Received 21 April 2008
Accepted in revised form 25 October 2008
Available online 13 November 2008
Keywords:
Stone protection
Super-hydrophobic surfaces
Silica nanoparticles
In the current study, we demonstrate that the modification of a commercial siloxane protective composition
by the addition of silica nanoparticles substantially enhances its protective efficiency and renders the treated
stone surface super-hydrophobic and self-cleaning. The extent of surface hydrophobization depends on
nanoparticle concentration and reaches a maximum value of ~160° at 1% w/v of nanoparticles for the case of
white Greek marbles (Naxos, Pentelic and Thassos) treated with the modified composition. The investigation
of the surface morphology by scanning electron microscopy (SEM) reveals the presence of micron-sized
protrusions (10–100 μm in diameter) formed by nanoparticle aggregates consolidated by the siloxane
polymer. The diameter and surface density of the protrusions depend on nanoparticle concentration. The
developed nanostructure of the protrusions was observed by atomic force microscopy (AFM). The nano-
dimensions of the silica particles are essential for the superhydrophobization of the treated marble surfaces.
In the case of micron-sized silica particles that were mixed with siloxane and were applied accordingly on
similar white Greek marbles, the superhydrophobic effect was not achieved and the observed water contact
angles were substantially lower.
In the event that hydrophobicity is not the sole parameter of optimal stone and stone-monuments protection,
other important parameters, such as water vapor permeability, water capillary absorption and stone color
alterations, were also investigated and their dependence on nanoparticle concentration was established.
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
Condensed water is one of the main factors for stone's and stone-
based monuments' weathering and decay [1]. Water can cause stone
deterioration through cycles of freezing and thawing inside the pores
of the stone or by intraporous crystallization of the salts transferred
by water from the environment. Moreover, different air pollutants [2]
can also be transferred and cause stone decomposition, e.g. by acid
erosion, or stone color alteration thus changing the aesthetic
appearance of the monument. Nowadays, the protection of stone
monuments from natural weathering is pursued by the application of
water-repellent polymer films on the stone surface. The polymer films
reduce the surface tension of the substrate and increase its water
repellent properties. A large number of low surface tension polymer
films have been synthesized and evaluated during the last decades.
Acrylics, fluorinated acrylic polymers, fluoropolyethers and siloxanes
are some of the coatings that have been studied [3–9]. Acrylics and
siloxanes have proved to be more suitable and are widely applied
nowadays, but they still do not satisfy all the demands for an ideal
protective coating for stone monuments and especially those of
cultural heritage value. The partially fluorinated and perfluoropoly-
mers were also investigated, since the introduction of fluorine atoms
substantially reduces the surface energy of the polymer film [1,10–12].
However, even perfluoroethers with the lowest surface energy of the
film formed can assure water contact angle only up to 120° [13].
Hydrophobicity can be further enhanced by an increase in surface
roughness, since surface energy and surface roughness are the
dominant factors that determine surface hydrophobicity. There are
two possible ways to enhance the surface roughness of a coating.
Firstly, by increasing the roughness of the substrate (e.g. by plasma
treatment [13,14]), as surface topography significantly affects film
topography. This strategy usually demands either special equipment
or complex process control and is certainly not appropriate for the
protection of stone monuments.
The second approach is to increase the surface roughness without
affecting the substrate. In our previous work, silica nanoparticles were
dispersed in solutions of poly(methyl methacrylate) (PMMA) and
commercial polyalkylsiloxane (Rhodorsil 224) and the suspensions
were sprayed on glass surfaces giving them super-hydrophobic
properties (water contact angle N150°)[15].
In the present work, we have applied the modified polyalkylsilox-
ane compositions on different Greek white marbles widely used for
buildings and stone-monuments construction in the past and now.
Surface & Coatings Technology 203 (2009) 1322–1328
⁎ Corresponding author. Tel.: +30 2410 565065; fax: +30 2410 565054.
E-mail address: atsakal@med.uth.gr (A. Tsakalof).
0257-8972/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.surfcoat.2008.10.041
Contents lists available at ScienceDirect
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journal homepage: www.elsevier.com/locate/surfcoat

2. The application resulted in super-hydrophobic treated marble sur-
faces. This is attributed to the presence of nanoparticles, which affect
the surface morphology. The surface morphology was examined by
SEM and AFM. In addition, water vapor permeability, water capillary
absorption and stone color alterations were measured and their
dependence on the nanoparticle concentration was established. These
measured quantities are additional parameters necessary to be con-
sidered for the optimal protection of stone and stone-based monuments.
2. Experimental
Three types of white Greek marbles were used as substrates: Pentelic
marble (98% calcite, 2% quartz, open porosity 0,21%), Naxos marble (98%
calcite, 2% quartz, openporosity 0,12%) and while marble of Thassos (86%
dolomite, 12% calcite, 2% quartz, open porosity 0,20%). The marble
specimens were squared blocks of around 2.5×2.5×1 cm cut from one
marble plate acquired from domestic marble supplier. Before treatment
the marbles were washed with deionized water and acetone. The surface
of the specimens was not abraded. The root-mean-square (rq) surface
roughness of the acquired marble specimens was measured by AFM at
20×20 μm scan areas and was found to be 1±0.1 μm for Pentelic marble,
1.2±0.1 μm for Naxos marble and 0.9±0.1 μm for Thassos marble.
Hydrophilic white-colored silica nanoparticles (Silica, fumed
powder, Aldrich) with 7 nm mean diameter were mixed with 7% w/v
polyalkysiloxane solution in white spirit (Rhodorsil Hydrof, Rhodia
Silicones, Italy). Different percentages of SiO2 nanoparticles (0, 0.1, 0.3,
0.5, 1, 1.5, 2% w/v) were used for mixing with the above polymeric
material. The mixtures were stirred vigorously for 20 min in order a
homogeneous dispersion to be prepared. The as-prepared polymer–
particle mixtures were sprayed onto three samples of the same type of
marble through a nozzle of 733 μm using an airbrush system (Paasche
airbrush). The quantity of the spraying mixture was kept steady by
controlling the spray pressure (2.5 bars) and the optimal spray time
(2 s) selected on the basis of preliminary experiments. Preliminary
experiments showed that spray times N5 s result in poorly adhered
films. The treated specimens were subsequently annealed at 40 °C and
low vacuum (0.5 bars) overnight and then kept at room temperature
for 2 to 3 days until constant weight (±0.001 g). After the evaporation
of the solvent, investigation of the surface properties and evaluation of
stone protection efficiency were carried out. The results presented in
this work are the average of the results obtained on three different
samples of the same type of marble.
The effect of nano-silica was compared to that of micro-silica.
Hydrophilic white-colored silica microparticles (Silicon dioxide,
Sigma) with particle size 0.5–10 μm (~80% between 1 and 5 μm)
were mixed (1% and 2% w/v) with Rhodorsil and the composition was
applied on Pentelic marble in a previously described manner.
Water contact angle measurements were conducted using distilled
water and a Krüss DSA 100 contact angle measuring instrument. Five
droplets of water were delivered to different points of each specimen
and from a height sufficiently close to the substrate, so that the needle
remained in contact with the water droplet. Then, the delivery needle
was withdrawn with minimal perturbation to the drop [16]. The
volume of each droplet was 5–8 μl. The contact angle hysteresis was
calculated by the dynamic sessile drop method. The advancing/
receding contact angle (θa/θr) was the maximum/minimum angle
measured, while the volume of the droplet was increased/decreased
without increasing/decreasing the solid–liquid interfacial area. The
reported contact angle values are averages of five measurements.
Water contact angle hysteresis is defined as the difference between
advancing and receding contact angle.
For the vapor permeability, sample blocks were fixed on the top of
identical cylindrical PVC containers that were partially (1/2) filled
with water. Then, the containers properly sealed with stone lids were
placed in a climatic chamber, kept at R.H. 25% and at constant
temperature of 40±0.5 °C. The containers were weighted every 24 h. It
was assumed that the vapor flow through the stone had reached a
constant value when the difference between two consecutive daily
(24 h) weight variations, ΔMi–1 and ΔMi, was less than 5% [4,17]:
ΔMi−ΔMi–1
ΔMi
×100b5k ð1Þ
Under constant vapor flow the water vapor permeability was
evaluated as the mass of water vapor passing though the surface unit
(cm2
) in 24 h. Three consequent measurements with the interval of
24 h were made and the average value was used.
Capillary water absorption measurements were performed by the
gravimetric sorption technique [3]. The dried weighted stone block
was placed with the treated side on a filter paper pad (1 cm of
Whatman paper, No 4) partially immersed in distilled water. After 1 h
of experiment, the sample was extracted and after removing the water
drops with a wet cloth, it was weighted again to determine the
amount of water absorbed by capillary forces.
The effect of the polymer–particle mixtures on the optical
appearance of the marbles was evaluated with colorimetric measure-
ments. The same five homogeneous spot areas of 4 mm in diameter, of
each specimen were examined, before and after treatment. For this
purpose a portable reflectance spectrophotometer MiniScan®
XE Plus
(HunterLab Associates Inc, USA) was used. The results were evaluated
by the use of L⁎a⁎b⁎ coordinates of the CIE 1976 scale [18,19].
The morphologies of the surfaces were investigated by scanning
electron microscopy (SEM, Jeol-5900LV) and by atomic force micro-
scopy (AFM, Multimode IIId, Veeco Inc.) operated in tapping mode.
The dimensions of the surface structures were measured with NIH
software (National Institutes of Health, U.S.A.).
3. Results and discussion
3.1. Surface morphology
The morphology of the treated marble surfaces depends on the
nanoparticle concentration in the siloxane protective composition, as
it can be seen from the SEM images (Fig. 1). Without nanoparticles a
continuous siloxane film was formed (Fig. 1a), which followed the
marble block surface morphology and assured water contact angle of
110°. The addition of nanoparticles led to the formation of separated
protrusions on the treated surface (Fig. 1b–d). The size and the surface
density of the protrusions were proportional to nanoparticles
concentration. In particular, the marble's surface area covered by the
protrusions increased from 10% (0.1% w/v silica) to more than 50% (2%
w/v silica). These observations testify that nanoparticles are not
homogeneously dispersed in the polymer film, but they form
aggregates with average diameters dependent on the nanoparticles
concentration and ranging from 10 μm (0.1% w/v silica) to 100 μm (2%
w/v silica). The diameters of the aggregates were estimated from SEM
images using ImageJ NIH software. The aggregates have irregular
shape (Fig. 1e) with highly developed nanostructure, as it can be seen
from the AFM images of the aggregate's surfaces (Fig. 2). Fig. 2a shows
the surface morphology of a 5×5 μm scan area. The nano-scale
roughness is clearly observed in Fig. 2b, where the scan area is only
1×1 μm and the roughness (rq) is 44.9 nm.
Therefore, the addition of nanoparticles to the siloxane composi-
tion resulted in the formation of the superficial micro protrusions and
overall alteration of surface morphology and roughness. The dimen-
sions of the protrusions (μm-scale) and the developed nanostructure
(nm-scale) led to the development of a two-length scale, micro- and
nano-roughness on the surface of treated marbles. As it is well-known,
a two-scaled surface roughness strongly amplifies surface hydro-
phobicity [20]. Thus, roughness at nano- and micron-scale has been
considered responsible for the development of super-hydrophobic
and self-cleaning properties on the Lotus leaf (“lotus effect”) and on
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3. other natural materials. The Lotus leaf is covered by many papillae
with their diameters in the range of 3–10 μm, while these papillae are
covered with smaller protrusions in the nano-scale [21,22]. Water
droplets are present on it with a nearly spherical shape and can roll off
easily with a slight vibration.
3.2. Water contact angle
The marbles are hydrophilic materials. The static water contact angle
(SCA) on the untreated marbles was 40–50°. Hydrophobization of
marble surface with unmodified siloxane composition (Rhodorsil
Hydrof) increases the SCA up to 110°. The addition of silica nanoparticles
further increases the hydrophobicity of the surface and for particle
concentrations N1% w/v, it renders the surface super-hydrophobic
(SCA N150°, Fig. 3). Maximum hydrophobicity (SCA ~160°) for all
marbles treated is achieved at 1% w/v of silica and further increase in
nanoparticles concentration does not enhance the surface hydrophobi-
city any more (Fig. 3). It is important to note that for concentrations
lower than 1% w/v, where the aggregates cover only relatively small
part of the substrate surface (Fig.1b), the observed water contact angle
depends on the marble nature (Fig. 3). For particle concentration
higher than 1% w/v, the water contact angle is independent of the
substrate. Obviously, at these concentrations, the surface density of
silica-siloxane aggregates is sufficient to abrogate the water contact
with the substrate. As it was calculated from the SEM images, at the 2%
w/v concentration the silica-siloxane aggregates cover more than 50%
of the substrate surface (Fig.1d). Interestingly, the application of 2% w/v
modified composition on other substrates assured the same level of
their hydrophobization (SCA=162°±0.5°) independently of the sub-
strate nature (glass, Si wafer, silk fabric, plywood, aluminium) [15]. The
observed similarities in hydrophobic behavior of different substrates
treated with 2% w/v modified composition are the consequences of the
same physical reason and the result of the formation of a superficial
silica-siloxane protective layer that eliminates the water droplet
contact with the substrate. This can be the consequence of the
heterogeneous wetting regime in which a water droplet sits on a
mixture of trapped air and solid silica-siloxane protrusions (Cassie-
Baxter regime [23]).
Fig. 1. SEM images of the Pentelic marble surface treated with Rhodorsil and (a) 0%, (b) 0.1%, (c) 1%, (d) 2% w/v silica nanoparticles and (e) of a single protrusion formed on the marble
treated with Rhodorsil and 2% w/v silica.
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4. It should be underlined that the nano-dimensions of silica particles
are important for the superhydrophobization of the marble surface. In
the case of silica particles with micro-dimensions, the superhydro-
phobic effect was not achieved and substantially lower contact angles
were observed. Thus, in the case of Rhodorsil modified by silica
microparticles (mean diameter 3μm) and then applied on Pentelic
marble, the observed SCAwas 120.5±5° and 132±4° for 1 and 2% w/v of
silica, correspondingly. These values are substantially lower than those
observed in the case of Rhodorsil modified by the same concentrations
of silica nanoparticles: 161.3±1° and 162±1.5° for 1% and 2% w/v of
nanosilica, correspondingly. Apparently, the nano-dimensions of silica
particles are essential for the formation of the developed nano-
topography/nano-roughness of the protective layer. As we mentioned
previously, a two-length-scale, micro- and nano-hierarchical surface
roughness is important to achieve super-hydrophobicity.
Super hydrophobic, highly water repellent properties of the
treated marble surfaces were as well confirmed by the investigation
of water contact angle hysteresis, which is more adequate for the
indication of hydrophobicity [24,25]. A highly water repellent surface
is characterized by low water contact angle hysteresis and thus, a
water droplet can move with little applied force and roll off easily. In
our case, at elevated silica concentrations (N1%w/v), the advancing
and receding contact angles were higher than 150° and consequently,
the water contact angle hysteresis was low. In particular, in the case of
Pentelic marble treated with 1% w/v modified composition the
hysteresis was 7° (θa/θr: 162°/155°). For 2% w/v particles, the
hysteresis was reduced to 5° (θa/θr: 164°/159°). As a consequence, a
water droplet easily rolls off from the surface, as it can be seen from
consecutive images (1–16, Fig. 4) of a water droplet on the treated
Pentelic marble surface. The water droplet was left from the needle
and after bouncing on the surface, it finally rolls off.
3.3. Water capillary absorption
It should be mentioned that even in the case of hydrophobic
surfaces the water can invade the stone by capillary absorption and
that protection against such invasion is part of the overall stone
protection strategy [3]. Previous works have demonstrated that the
water absorption is very rapid and most of the amount of the water is
absorbed within the first 20 min [3,26]. Therefore, the water capillary
absorption experiment time was set to be 1 h. The reduction of water
capillary absorption (RCA) is defined as:
RCA =
mu−mt
mu
×100 ð2Þ
mu: the mass of the water absorbed by the untreated substrate
mt: the mass of the water absorbed by the treated substrate.
An ideal protective coating should eliminate water absorption by
capillarity. The water absorption by capillarity of the untreated marbles
was 3–5 mg/cm2
h. When pure polymer was sprayed on the marbles,
the water absorption reduced by 69.4±1.4–75.4±1.5% (Fig. 5). At low
particle contents, from 0 to 1% w/v, a gradual decrease in the reduction
of water capillary absorption is observed. The minimum reduction of
the water capillary absorption was 40±2% in the case of 1% w/v particle
concentration on the marble of Thassos. In this range of particle
concentration (0.1–1% w/v), the aggregates formed are sparsely
dispersed on the surface of the marbles (Fig. 1) covering less than
25% of the total surface area. At the same time, a part of the siloxane
polymer is obviously consumed for the silica-siloxane aggregates
consolidation. As a result, the polymer film in-between aggregates
becomes thinner or even disrupted and this fact leads to the decrease in
the efficiency of stone protection from water capillary absorption.
At elevated particle concentration (N1% w/v), the coverage of the
stone surface by the hydrophobic aggregates is significantly higher,
Fig. 3. Water contact angle vs. concentration of silica nanoparticles. Photograph of water
droplets on Pentelic marble treated with 2% w/v silica nanoparticles is included.
Fig. 2. AFM images (a) scan area 5×5 μm (b) scan area 1×1 μm, on the surface of the protrusions formed on the Pentelic marble surface treated with Rhodorsil and 1% w/v SiO2. At the
upper side of each image the root-mean-square roughness (rq) is presented.
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5. reaching a maximum of about 50% at 2% w/v (Fig. 1d). So a dense
superficial, highly hydrophobic silica-siloxane composition layer is
formed on the surface of the marbles. This results in the decrease of
water capillary absorption up to 89.5±0.7–96±1% (Fig. 5). The
achieved reduction was substantially better than that achieved by
the application of the non-modified siloxane composition.
The results obtained also demonstrate that the development of
super-hydrophobicity on the stone surface does not necessarily assure
optimal stone protection against water invasion. For example, in the
case of polymer–particle composite films with 0.5–1% w/v particle
content, the marbles are super hydrophobic, while the reduction of
water capillary absorption is lower compared to the reduction
obtained when pure polymer is applied on the marbles.
Under real outdoor conditions, stones can be exposed to
condensed water for longer than 1 h, time usually used in experiments
for the estimation of water capillary absorption. For this reason, we
also investigated the reduction in water absorption by capillary for
24 h exposure of the treated Pentelic marble samples. For this
prolonged exposure the reduction in water capillary absorption by
pure siloxane film was only 20±1%, while by the application of 2% w/v
silica-siloxane composition the reduction was substantially higher
and reached 50±2%. These data once again demonstrate the enhanced
efficiency of silica-siloxane composition for stone protection.
3.4. Water vapor permeability
The water vapor transmission rates through the stone should not
be reduced after treatment, assuring the proper vapor regime inside
the stone and the building. The impermeability of the stone protective
coating to water vapor can lead to water condensation just under-
neath the protective layer and to subsequent stone decay by the above
described condensed-water action. The reduction of water vapor
permeability is inevitable, since it is an immediate consequence of the
water repellence properties of the protective layer. However, the
lowest possible decrease is pursued. The reduction of water vapor
permeability is defined as:
RVP =
mu−mt
mu
×100 ð3Þ
mu: the mass of the vapor which penetrates the untreated substrate
mt: the mass of the vapor which penetrates the treated substrate.
The vapor permeability through the untreated marbles was 0.11–
0.14 g/cm2
d. When pure polymer was applied to marbles, the
reduction of the vapor permeability was 10±1%–12±1% depending
on the substrate (Fig. 6). The addition of silica nanoparticles to the
composition further reduces the vapor permeability with maximum
reduction of 40.3±0.5% at 2% w/v of nanoparticles. It is noteworthy
that the vapor permeability decreases with increasing the particle
concentration in the polymer–particle composite film. The reduction
of water vapor permeability is obviously associated with the density of
the aggregates formed on the surface of the treated marbles. As the
coverage of the marble surfaces by the protrusions increases, the
vapor flow is inhibited. The reduction of the vapor permeability
caused by the polymer–particle composite films is not negligible, but
it is lower than the reduction reported for the fluorinated acrylic
copolymers [27].
Fig. 5. Reduction of water capillary absorption vs. silica nanoparticle concentration.
Fig. 4. Consecutive images of a water droplet bouncing and rolling off a Pentelic marble treated with Rhodorsil and 1.5% w/v silica nanoparticles.
1326 P.N. Manoudis et al. / Surface & Coatings Technology 203 (2009) 1322–1328

6. 3.5. Color alteration
The variation on the optical appearance of the treated marbles was
evaluated with colorimetric measurements. An ideal protective
coating should not have any impact on the optical appearance of the
stone. The total color difference (ΔΕ⁎) on the same spot of the same
sample before and after treatment is defined as:
ΔET =
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ΔLT2
+ ΔaT2
+ ΔbT2
p
ð4Þ
L⁎: brightness (0 for black–100 for white)
a⁎: red–green component (positive for red and negative for the green
colors)
b⁎: yellow–blue component (positive for yellow and negative for the
blue colors).
The total color difference results are presented in Fig. 7. At low
particle concentration, the color difference due to the polymer–
particle composite films is lower compared to the color difference
caused by pure siloxane. At elevated particle content, the dense surface
morphology results in an enhancement of ΔΕ⁎, which increases with
particle concentration in the polymer particle composite film. The
maximum total color difference is caused by the polymer and 2% w/v
silica particles. The maximum ΔΕ⁎ obtained on the marbles is
comparable with the ΔΕ⁎ caused by the application of various pure
polymer films on the same marble [3].
It is noteworthy that the color difference is mainly caused by
changes in the brightness (L⁎) of the treated marbles. In particular, the
pure polymer film slightly reduces the brightness of marble surface in
accordance with the previous results [3]. On the contrary, due to the
high brightness value of nanoparticles itself (L⁎=88.0), their addition
restores and, at higher concentrations, even enhances the brightness of
the marbles' surface. This is especially evident for the relatively “dark”
marble of Thassos, where the brightness of the untreated sample
increased from 69.1±1 to 76±0.8, in the case of 2% w/v nanoparticles.
The red–green chromatic component a⁎ (positive for red and
negative for green colors) remained practically unaltered. Similarly,
for the yellow–blue chromatic component b⁎ (positive for yellow and
negative for blue colors), no significant variations were observed, that
is, no yellowing was measured.
The acquired results demonstrate that the addition of nanoparti-
cles affects mainly the brightness of treated white marble surfaces
without affecting the other color components (a⁎ and b⁎).
3.6. Outdoor exposure
The protective treatments on stones, which include polymers, are
usually sensitive to environmental parameters like UV irradiation. The
photochemical stability of the polymers used for the protection of
monuments has been extensively studied mainly under artificial light
irradiation [28–30]. In an environmental chamber, however, para-
meters like rainfall and dust deposition cannot be studied and
atmospheric pollution can be simulated to a certain degree only. For
this purpose and in order to evaluate the durability of the protective
treatment, a Pentelic marble treated with siloxane and 2% w/v
nanoparticles was exposed to outdoor conditions. After 5 months of
outdoor exposure, water contact angle measurements were carried
out on the marble. The static water contact angle was reduced from
162±1.5° to 150±1°. The obtained results show that, despite the
reduction in water contact angle, the treated marble surface remained
super-hydrophobic and thus, they demonstrate the durability of the
suggested treatment.
4. Concluding remarks
Our data demonstrate that the modification of the commercial
siloxane protective composition by the addition of silica nanoparticles
with nominal diameter 7 nm can substantially enhance the efficiency
of the composition for the protection of stone buildings and
monuments. The application of the modified composition on Greek
marble substrates (marbles of Naxos, Pentelic and Thassos) render the
treated surfaces superhydrophobic with highly water-repellent and
self-cleaning properties. The enhancement of the protection efficiency
depends on the nanoparticle concentration. For nanoparticle concen-
trations N1% w/v (e.g. 1, 1.5 and 2%), the acquired hydrophobic
properties are independent of the nature of the substrate and are
characterized by water contact angle of about 160° and by contact
angle hysteresis of 5°. The observed effect is explained by changes in
the surface morphology of the treated marbles caused by the addition
of nanoparticles. Initially (0% nanoparticles in siloxane protective
composition), there is a thin polymer protective film, which follows
the morphology of the substrate surface. The addition of nanoparticles
leads to the formation of superficial micron-sized protrusions with
highly developed nanostructure. The size (10–100 μm) and the surface
density of these protrusions depend on the nanoparticle concentra-
tion in a proportional way. Apparently, the alteration of the surface
morphology changes the wetting mode of the surface from initially
homogeneous (Wenzel model) to heterogeneous (Cassie-Baxter
model) mode, where substantial amount of air is trapped in-between
the water droplet and the substrate. The latter mode is often referred
Fig. 6. Reduction of water vapor permeability vs. silica nanoparticle concentration.
Fig. 7. Total color variation (ΔΕ⁎) vs. silica nanoparticle concentration.
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7. as “slippery” regime and is characterized by a small contact angle
hysteresis and thus a small sliding or tilt angle [31,32]. In the approach
reported here, the contact angle hysteresis reached very low values of
about 5° on all the marbles treated with the modified composition of
more than 1% w/v nanoparticles.
The nano dimensions of silica particles are essential for the
superhydrophobization of the treated marble surfaces. In the case of
micron-sized silica particles (mean diameter ~3 μm) that were used
and applied by the same methodology, the superhydrophobic effect
was not achieved and the observed water contact angles were
substantially lower. We suggest that the nano dimensions of particles
are important for the formation of the developed nanostructure/nano
roughness of the protective layer. Nano-roughness together with
micro-roughness (assured by silica micro-protrusions) is equally
important for the development of surface super-hydrophobicity.
Concerning the other parameters, which are also important for the
efficient protection of the stone monuments and buildings, namely the
reduction of water capillary absorption and the stone permeability to
water vapor, the optimal concentration for the nanoparticles can be
indicated. At this concentration, namely 1.5% w/v of nanoparticles, the
modified Rhodorsil composition offers sufficiently enhanced, optimal
protection for marble monuments and buildings. This modified
composition renders their surface super-hydrophobic and self-clean-
ing, considerably reduces (from 89.5±0.7 to 96±1%) water capillary
absorption and, albeit decrease water vapor permeability, still
maintains it at an acceptable level.
The suggested modification is simple, cost-effective and an
efficient approach for the improved protection of stone monuments
and buildings from decay and deterioration.
Acknowledgements
This research was supported by PENED 2003 program that is co-
financed by E.U.-European Social Fund (75%) and the Greek Ministry of
Development-GSRT (25%). The support of Greek State Scholarship
Foundation to P.M. is also gratefully acknowledged. The authors are
also grateful to Dr. S. Marras for the SEM images.
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