APPLICATION OF TiO2 BASED COATINGS ON STONE SURFACE OF INTEREST IN THE
FIELD OF CULTURAL HERITAGE
L.Luvidi1, G.Laguzzi1, F. Gallese1, A. M. Mecchi2, I. Nicolini2, G. Sidoti3
1
Istituto di Metodologie Chimiche - CNR
Via Salaria Km 29.300 - 00016 Monterotondo (Roma), Italy
2
Istituto per la Conservazione e la Valorizzazione dei Beni Culturali - CNR
Via Salaria Km 29.300 - 00016 Monterotondo (Roma), Italy
3
Istituto Superiore per la Conservazione ed il Restauro
Piazza San Francesco di Paola, 9 - 00184 Roma, Italy
ABSTRACT
In the frame of the activities related to the preventive conservation and maintenance of Cultural
Heritage, the application of titanium dioxide based treatments on stone surfaces could offer an
advanced contribution, limiting the cleaning interventions. The present work has been carried out
to verify the effectiveness of these products and the absence of damages on the surfaces. Stone
specimens, as calcareous matrixes with low porosity and different colours, were treated with two
different TiO2 based products. The first one, commercially available, is a water solution of TiO 2
nanoparticles. The second one, under experimentation, consists in a TiO2 functionalized
polisiloxane. The homogeneity of distribution on the surfaces was verified by SEM-EDS and the
water repellent property of the products was evaluated by measurements of the static contact
angle. Since an important requirement for the application of coatings on stone surfaces of artistic
and historical interest is the maintenance of the aesthetic characteristics, colorimetric tests were
performed on the treated materials. Furthermore, to check the photo-catalytic properties of the
TiO2 based products, treated specimens coated with an organic colorant, were submitted to UV
irradiation and the relevant changes of colour were measured by a spectrocolorimeter.
The obtained results are promising for a future application and a wider experimentation will be
planned to investigate the effectiveness of these products and their harmfulness. This will include
the most common building stones in Mediterranean area exposed to polluted environment.
KEYWORDS: Titanium dioxide, Self-cleaning treatment, Maintenance, Stone treatments.
1. INTRODUCTION
In the last years based on titanium dioxide, innovative materials have been developed. Because of
their specific characteristics, these products are able to induce the photo-decomposition of
inorganic (e.g. NOx) and organic pollutants (e.g. benzene and other aromatic compounds or
carbon black) adsorbed on their surface [1]. For this reason the TiO2 photo-catalysis produces antipollution and self-cleaning effects [2,3 and 4].
In particular a stone surface coated with titanium dioxide can maintain itself clean, under ultraviolet
illumination, since the TiO2 photo-catalyst acts against the soiling processes.
This “self-cleaning technique” is obviously of great value since it works utilizing the mere solar light
and contributes in the decreasing the frequency of the conservative treatments with an evident
benefit in term of costs of maintenance.
TiO2 in the allotropic form of anatase, well known for its photo-catalytic properties, induces a
superhydrophilicity of the surface when exposed to UV light. This property allows removal of
degradation substances by formation of an uniform water layer. On the other hand the treated
surfaces are more exposed to the degradation agents such water and acidic/alkaline water
solutions. This is, of course, a negative aspect for materials of interest in the field of Cultural
Heritage.
Nowadays several TiO2 based products, as mineral and organic coatings, are available to be used
in new building [5,6]. TiO2 based products have been scarcely investigated on stone surfaces of
artistic and historical interest. Our aim is the experimentation of these innovative products for a
possible application in the field of Cultural Heritage.
For an effective use of TiO2 based products, on surfaces of historical buildings and monuments,
these innovative materials have to be submitted to different tests to check: a- the maintenance of
the aesthetic characteristics; b-the absence of harmfully (i.e. possible side effects produced during
the photo-catalytic activity); and c- the compatibility with other conservative treatments.
Concerning the possible side effects of the photo-catalytic reactions, a particular attention has to
be placed on the investigation of the degradation mechanism of NOx [7,8]. This molecule, in fact,
under the catalytic action of TiO2 can lead to the formation of calcium nitrate. In this case,
depending on the relative amount of Ca(NO3)2 formed, the stone could suffer all the problems
related to the presence of soluble salts on the surface.
In the present paper, the investigated TiO2 based products include a water solution of titanium
dioxide in form of nanoparticles and a TiO2 functionalized polisiloxane. In the last product the
combination with a water repellent is useful because it limits the water penetration. Colorimetric
test results, hydrophilicity changes and photo-catalytic activity are reported for the stone
specimens before and after the treatment with the TiO2 based products. The photo-catalytic
efficiency of the TiO2 based products has been evaluated by monitoring the discolouration of
rhodamine B, an organic dye, applied to the stone surface [9]. The homogeneity of distribution on
the surfaces has been verified by SEM-EDS.
2. EXPERIMENTAL
2.1 Stone materials and products
The investigated stones have been: white marble, red limestone and black limestone. They have
been characterized by XRD and by mercury porosimetry. White marble samples contain only
calcite and the total porosity (%) is 0.95. The red limestone specimens contain both calcite and
dolomite and the total porosity (%) is 0.33. The black limestone samples contain only calcite and
the total porosity (%) is 0.40. The tested materials show a similar mineralogical composition and a
similar total porosity.
Two titanium dioxide based treatments have been used. The treatment A (TA) consists of a
preliminary application of the primer TiOXOguard, followed by the application of TiOXOclean.
These products have been purchased by ACEP srl. Both the products are water solutions of TiO 2
nanoparticles. TiOXOguard does not exhibit photo-catalytic properties, TiOXOclean containing
TiO2 in the allotropic phase Anatase, is an active photo-catalytic product. The treatment B (TB)
consists in the application of a TiO2 functionalized polisiloxane which is a new formulation under
investigation.
The TiO2 based products have been applied by brush on stone samples, previously dried in oven
until the weight of the sample was constant.
2.2 Experimental procedure
The following tests have been carried out on the specimens:
2.2.3 Colorimetric analyses have been carried out according to the protocol Normal [10] by
Spectrophotometer Konica Minolta CM 2600d in total reflectance (measurement area 800
MAV). The colour changes (E) have been evaluated by the L*a*b*system (CIE 1976).
Thirty measurements have been carried out on each stone sample before and after the
TiO2 based treatments.
2.2.2 The determination of the contact angle of stone specimens treated and untreated has been
performed, according to UNI-Normal [11], by using a Costech instrument. For each sample
the obtained data have been the average of ten measurements. In the case of red and
black limestone samples, coated with the product A, it was not been possible to measure
the contact angle of the drop since it was absorbed in a few seconds. The contact angle for
2.2.3
2.2.4
white marble samples coated with the product A has been measured although for a short
time (10 sec).
Photo-catalytic activity evaluation by rhodamine B based colourimetric test has been
carried out on untreated and treated samples stained with rhodamine B and placed
horizontally with the light source located 50 cm above the surface of the specimens. The
light source was a 125 W UV lamp Zp type purchased by Helios Ital Quartz. The
discoloration of the rhodamine B has been measured after 2, 4 and 20 hours by
Spectrophotometer Konica Minolta CM 2600d. For the photo-catalytic test only white
marble specimens have been investigated.
Morphological analysis and elemental distribution of the products on stone surface has
been carried out by SEM-EDS analysis using a EVO 60 Zeiss scanning electron
microscopy, equipped with a Lanthanum Hexaboride filament cathode. The investigation
has been conducted at variable pressure of 100Pa without any pre-treatment of the
samples, the images were obtained by backscattered electrons.
3. RESULTS AND DISCUSSION
3.1 Colorimetric tests
Different stone materials have been tested to evaluate the maintenance of aesthetic characteristics
before and after the application of the treatments A and B. The results of the colorimetric tests are
reported in Tab. I, II and III.
UT
TA
TB
L*
a*
b*
E
89,01 ± 0,67
-0,56 ± 0,04
0,53 ± 0,20
-
87,57 ± 0,89
-0,58 ± 0,03
0,04 ± 0,14
1,55 ± 0,85
88,26 ± 0,36
-1,44 ± 0,07
2,54 ± 0,19
2,12 ± 0,23
Tab. I - Colorimetric coordinates (L*a*b*) and colour variation (E) of white marble samples
untreated (UT) and treated with product A (TA) and product B (TB).
UT
TA
TB
L*
a*
b*
E
58,0 ± 1,49
5,83 ± 0,53
0,83 ± 0,46
-
60,80 ± 1,61
4,57 ± 0,67
-0,30 ± 0,26
3,32 ± 1,57
55,02 ± 1,44
6,02 ± 0,69
1,57 ± 0,30
3,29 ± 1,24
Tab. II - Colorimetric coordinates (L*a*b*) and colour variation (E) of red limestone untreated
(UT) and treated with product A (TA) and product B (TB).
UT
TA
TB
L*
a*
b*
E
52,63 ± 1,80
-0,62 ± 0,05
-2,10 ± 0,20
-
55,52 ± 0,96
48,35 ± 0,80
-0,65 ± 0,04
-0,56 ± 0,03
-2,56 ± 0,13 2,94 ± 0,94
-1,40 ± 0,14 4,34 ± 0,80
Tab. III - Colorimetric coordinates (L*a*b*) and colour variation (E) of black limestone samples
untreated (UT) and treated with product A (TA) and product B (TB).
White marble: from the inspection of Table I it is possible to observe that the E value obtained by
treatment A and B is about 2. This is lower with respect to the human eye possibility of detection.
Moreover considering the single chromatic coordinates, it is possible to observe, that the treatment
A produces a very small decrease of the b* coordinate. For the treatment B, a larger decrease of
the a* coordinate and mostly an increase of the b* coordinate is observed, consequently a shift
towards a yellow/green hue can be noticed.
Red limestone: data of Table II indicate that the E value, obtained by treatment A and B, is about
3.3. This is instead hardly above the human eye possibility of detection. Considering the single
chromatic coordinates, a decrease of both the a* and b* coordinates is observed for treatment A,
consequently is evident a shift towards a green/blue hue. For the treatment B, a decrease of the L*
value and an increase of the b* value are observed so a shift towards a yellow colour can be
noticed.
Black limestone: results of Table III show E values respectively 3 and 4.3 for the A and B
treatments. Considering the single chromatic coordinates, changes of the L* and b* values are
obtained for treatment A, therefore a colour variation towards a blue hue can be observed.
Changes in L* and b* values are also produced by the treatment B, in this case is obtained a
colour variation towards a yellow hue.
3.2 Contact angle
The results of the wettability test (Tab. IV) show that the treatment A (TA) makes the surface more
hydrophilic respect to the untreated surface. Treatment B (TB) gives a good hydrorepellence to the
stone material.
Contact
angle
White marble
UT
White marble
TA
White marble
TB
Red limest.
TB
Black limest.
TB
81,37 ± 5,76
37,64 ± 3,51
127,02 ± 3,42
132,04 ± 4,56
125,83 ± 3,39
Tab. IV - Contact angle measurements of stone samples untreated and treated; UT and TA white
marble values are obtained after only 10 s.
3.3 Photocatalytic activity
In Fig.1 have been reported the variations in the colorimetric coordinates *a as a function of time
for untreated stone specimens (rUT) and treated stone specimens (rTA and rTB) stained with
rhodamine B. In particular the a* coordinate has been evaluated since white marble samples used
for the test gave an initial pink coloration after application of the colorant. rTA samples show a very
strong decrease of the a* value in the first hours of illumination. The starting value of the a*
coordinate for the rTA samples is very high with respect to the a* value of the rUT and rTB
samples. After 4 hours the rate of decrease of the a* coordinate is similar for all the samples.
These results therefore indicate that photo-catalysis has been mostly effective within the first 4 h of
UV irradiation and depends on the product used. The treatment A, where the titanium dioxide is
present in nanoparticles, show a higher activity respect to the treatment B. rTB samples exhibit the
lowest dye absorption because of its hydrorepellent properties.
Although the rodamine B based colourimetric test is largely used, this is not fully representative of
physical/chemical phenomena that occur in the degradation processes of organic pollutants
present on the TiO2 treated stone surfaces. For this reason our future activity will be addressed
toward the examination of the real effectiveness of the photo-catalytic treatments under outdoor
exposure.
Fig. 1 - Variation of a* coordinate as a function of time of white marble samples untreated (rUT)
and treated with product A (rTA) and product B (rTB) stained with rhodamine B.
2.3 SEM-EDS analysis
The SEM micrographs and EDS maps of distribution of elements showed a different morphological
aspect of the surfaces treated with treatment A and treatment B and a different distribution of the
titanium. In Fig. 2 and 3 are reported a BSE-SEM image and a titanium distribution map of a red
limestone sample with the treatment A. The BSE-SEM image shows a bright regions with a flake
light morphology where the titanium signal is strong and a residual area where the calcium signal
testifies a calcite matrix as evidenced by the distribution element map (Fig. 3).
In Fig. 4, 5 and 6 are reported a BSE image and a titanium and silicon distribution maps of white
marble treated with treatment B. The images show regions with a glass like morphology where
titanium and silicon signals are strong. The residual area shows a strong calcium signal which is
representative of a calcite matrix.
Fig. 2 - SEM-BSE image (1000x) of a red limestone sample with treatment A.
Fig. 3 - Titanium distribution map on the red limestone surface with treatment A.
Fig. 4 - SEM-BSE image (2000x) of a white marble sample with treatment B.
Fig 5 - Titanium distribution map on the white marble surface with treatment B.
Fig. 6 - Silicon distribution map on the white marble surface with treatment B.
4. CONCLUSION
In this paper are reported preliminary analyses concerning the treatments (A and B) of stone
surfaces with two titanium dioxide based products. Both the A and B treatments do not produce
any colour variation which can compromise the aesthetic characteristics of the stone surfaces. The
treatment B shows an acceptable homogeneity of distribution on the surfaces and gives them a
good water repellent property. On the contrary stone materials submitted to the treatment A seem
to be more hydrophilic with respect to uncoated materials. This aspect is potentially harmful for
stone surfaces because the materials can easily adsorb soluble salts and acidic/basic compounds
from environment. Concerning the photo-catalytic properties, the treatment A shows a greater
photo-activity with respect to the treatment B, due to the nanosized characteristics of the titanium
dioxide particles. That is, the TiO2 is more likely available to exert its photo-catalyst nature.
According to these results treatment B (not yet commercially available) could be an interesting
response to the problem of conservation of stone materials in the field of Cultural Heritage even
though the resulting photo catalytic activity should be increased. The treatment A produces a good
photo catalytic activity, does not produce any colour variation but it will be necessary to evaluate
the possible damages related to the hydrophilic effects produced on the stone surfaces.
These results are promising for a wider experimentation consisting in the examination of the
effectiveness and the harmfulness of the TiO2 based treatments on the most used stones in the
Mediterranean area.
Acknowledgments
The Authors are grateful to Dr. Maurizio Calvesi of the CNR-IBAM (Lecce, Italy) for his support in
the angle contact measurements.
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