Influence of Titanium Dioxide Pigments on the Photodegradation of Paraloid Acrylic Resin
Panayotis Spathis, Evi Karagiannidou and Anastasia-Eleni Magoula
Marble and stone used as building materials are susceptible to environmental damage. Polymers
can be used as carriers of suitable pigments for the protection of the surface of a monument. Tlie
photodegradation of a series of Paraloid B72 films containing titanium dioxide (TiOj pigments has
been studied under UV irradiation. Two kinds of TiO2, anatase and anatase/rutile mixture, were
used in different concentrations, 5, 15 and 30% w/w. The chemical and molecular changes
induced by the irradiation treatment were followed by Fourier transform infrared spectroscopy, gel
permeation chromatography, and solubility measurements. The presence of anatase pigment
significantly improved the photostability of Paraloid films.
INTRODUCTION
Marble and stone have always been considered the best materials for the building of monuments
and therefore a substantial part of our cultural heritage is made of them. They both deteriorate on
aging and therefore monuments in marble and stone are subject to decay [1]. There are several
causes for deterioration but one of the greatest dangers is weathering, caused by climatic changes
and air pollution.
Building marbles and stones are susceptible to various atmospheric factors causing their
destruction, especially in the Mediterranean basin where the marine salts are a permanent cause of
natural pollution, not only on the coast but also further inland. Also, in a changeable environment,
the surface of a marble or stone can be colonized by micro- and macroflora, another important
damaging factor.
Consolidation is intended to increase the resistivity and coherence of damaged marble or stone. In
order to obtain a compact and stable material which can withstand environmental effects, a good
consolidating agent has to meet several criteria. It should be stable; it should penetrate to a
sufficient depth and improve the mechanical properties of the object; it should be tough and
adhesive to marble/stone; and it should not change the colour or the appearance of the material [2].
The consolidating agents most often used are organosilicone compounds, epoxy resins, polyurethanes, silicones, fluorinated resins, polyester resins, acrylic polymers and copolymers. Polymers
with no pigments have been widely used and are still used today but to a smaller extent. These
consolidants, after five to 15 years of use, have been proved by many researchers [3-5] to have
damaging effects on the monuments. However, the polymers can be used as carriers of suitable
pigments for the protection of the surface of a monument. In order to choose the more suitable
polymers for this purpose, one has to consider those most durable to temperature, to light, to atmospheric chemical attack, to hydrolysis, and also the most reversible [3]. Therefore one is confined to
the acrylic polymers as carriers of pigments, because they are the only ones which are reversible.
The issue of reversibil-
ity of stone treatment has been generally questioned [6], in particular for Paraloid B72 in mixture
with silicone resins [7]. Certainly, in many countries, both non-reversible and unmixed acrylics are
used which accelerate the deterioration of the 'protected' material.
Polymers based on acrylic and methacrylic esters are widely used in the formulation of paints,
surface coatings and adhesives because of their good characteristics of film forming and mechanical
properties, optical clarity and overall stability. One particular niche-market of acrylic resins that
specifically exploits their good adhesion and water-repellent properties relates to the consolidation
and protection of different substrates in monuments and works of art. For such materials, durability
is fundamental for the long-term efficiency of the protective/consolidative treatment, particularly in
the case of outdoor applications [8]. The long-term efficiency of the protective treatment is mainly
influenced by the direct action of sunlight in promoting oxidation reactions, whose effects may be
exacerbated by temperature, moisture, and especially by the presence of atmospheric pollutants [9].
It has been proved that the presence of n-semiconductors in an acrylic polymer can protect it from
ultraviolet radiation, improve the coefficient of thermal expansion in comparison with the pure
polymer, repulse the negatively charged dispersed particles and a variety of microorganisms and,
finally, improve the resistance of the stone or marble to corrosion by mechanical strain [3]. The
effect of the pigment on the photostabilization of polymers has been extensively reported in the
literature [10-12].
The role of titanium dioxide (TiO2) pigments in the photodegradation process of polymers has not
been completely elucidated until now and conclusions are contradictory: does TiO2 act
predominantly as a physical UV absorber (Figure 1) and protect the polymer from UV irradiation,
or is it a photocatalyst that is activated by UV in combination with oxygen and water and
accelerates degradation? The investigations of polymer scientists support the first view, while
application/technological experience is mixed [11, 12]. The authors have investigated the effect of
titanium dioxide pigments on the photostabilization of the widely used consolidant Paraloid B72. A
series of thin films of Paraloid B72 containing different concentrations of TiO2 have been tested
with artificial UV irradiation and the structural and molecular changes occurring during treatment
have been followed by Fourier transform infrared (FTIR) spectroscopy, gel permeation
chromatography (GPC) and solubility
Figure 1 Absorption spectra of anatase (-) and rutile (—}.
determinations. This work has been done within the frame of a research programme for the
restoration and conservation of the monuments of the historic cemetery of Drapanos in Argostoli,
the capital of the Greek island of Kefallonia.
EXPERIMENTAL
Materials
The consolidant material investigated is produced by Rohm and Haas (USA) and is traded under the
name of Paraloid B72. It is a copolymer of methyl acrylate (MA) and ethyl methacrylate (EMA), in
the molar ratio 30:70 (Scheme 1). Some characteristics of this copolymer are reported in Table 1.
The titanium dioxide used was of two kinds: anatase (Hombicat UV100 250 m2g-1) and a mixture of
anatase and rutile (P25 Degussa 65 m2g-1) in the ratio 35:65% w/w. Rutile and anatase have the
same chemical formula but they differ distinctly from each other in their physical properties. Their
crystal lattices are different, as are their forms. Naturally occurring anatase
Scheme 1
Table 1
Molecular and thermal characteristics of Paraloid B72 used (provided by supplier)
(a) Mn : number average molecular weight
(b)Mw : weight average molecular weight
usually forms almost regular octahedrons. Rutile forms acicular prismatic crystals, often double
crystals [10].
Preparation of the films and photo-irradiation
Five solutions of Paraloid B72 in acetone (propanone) were prepared with three different
concentrations of anatase and anatase/rutile mixture (5, 15 and 30% w/ w in relation to Paraloid
B72). The samples were treated in the form of thin films, cast from acetone solutions and dried
under atmospheric air. Glass slides (5 X 5cm) were used for the formation of films with a thickness
of ~10μm. The temperature during irradiation was 25°C. The slides were irradiated for six and 12
hours, at a distance of 5cm from the UV lamp. Photo-irradiation was carried out using a mercury
medium pressure lamp (230 < λ< 440nm, 125W).
Measurements
The infrared spectra were registered with a Bruker Company IPS 113v spectrophotometer
(resolution 2cm-1, number of scans 32).
Molecular weight distributions were determined before and after irradiation on the soluble fraction
by gel permeation chromatography (GPC). The system comprised a Marathon Series III HPLC
pump from Rigas Labs, a PL-DCU data capture unit and a PL-EMD 950 evaporative mass detector,
both from Polymer Laboratories. A PL gel Sum mixed D GPC column (Polymer Labs, UK) with
200-400,000 molecular weight range was used for the separation. Sample solutions, approximately
0-2% (w/v) concentration, were prepared in distilled tetrahydrofuran, which was also the
chromatographic eluent. Before injection, the solutions were filtered on 0-2um PTFE membrane
syringe filters. The eluent flow-rate was set at 1cm3min-1. Column calibration was performed with
'narrow' standard samples of polystyrene.
For the evaluation of solubility, the irradiated films were first put into glass filters and weighed.
Then they were carefully washed with tetrahydrofuran (THF), to dissolve all the soluble polymer,
and filtered under vacuum. After filtration, the glass filters were put in a vacuum oven for 24 hours
at 50°C. The amount of polymer remaining on the glass filter, determined by the difference between
the initial and final weight, was used to calculate the percentage of insoluble fraction of films in
THF.
RESULTS AND DISCUSSION
FTIR spectra
The FTIR spectrum of Paraloid B72 before photo-irradiation is shown in Figure 2. The most
relevant changes observed in this spectral region after photo-irradiation by UV light are:
Figure 2 FTIR spectrum of Paraloid B72 before photo-irradiation (2000-400cm-1).
Figure 3 FTIP spectra of Para/oid B72 (1600-900cm-1) (a) before irradiation and (b) after UV
irradiation for 12 hours; and of Paraloid B72 after UV irradiation for 12 hours (c) with anatase
15% w/w and (d) with anatase/rutile mixture 15% w/w.
1 A significant decrease of the peak at 1477cm-1, attributed to the asymmetric bending of the (Xmethyl group of the ethyl methacrylate monomeric unit, in comparison to the peak at 1447cm-1 of
the methyl group of the ester group (Figure 3).
2 The broadening at both sides of the carbonyl stretching absorption band of the ester group at
1726cm-1 and the appearance of a shoulder at 1780cm-1, which may be attributed to the formation of
y-lactones [1, 9].
According to the literature [1, 9], Paraloid B72, when irradiated by UV light, initially undergoes
scissions on the side ester groups of MA monomeric units, which lead mainly to the formation of
volatile molecules and γ-lactones (Scheme 2). Scissions of the main polymeric chain also take
place, with formation of lower molecular weight molecules, and cross-linking reactions by
recombination of macroradicals leading to higher molecular weight molecules. When ultraviolet
irradiation is used (λ<290nm) the main degradation pathway is chain scission, bringing the polymer
structure to complete fragmentation [1]. The light source used in this work emits predominantly at
254, 265, 297, 313 and 366nm. This high-energy radiation can cause extensive homolysis of all
types of a wide range of chemical bonds.
The IR spectrum of TiO2 absorbs at 500-800cm-1, so its presence in Paraloid samples does not
hinder observation of the spectral changes of Paraloid containing TiO2 after photodegradation.
Figure 4 shows the FTIR spectra of Paraloid samples containing 5, 15 and 30% w/w anatase, before
and after irradiation for six and 12 hours.
Scheme 2
Figure 4 FTIR spectra of Paraloid B72 with (left) 5%, (centre) 15% and (right) 30% w/w anatase
before and after irradiation by UV light for six and 12 hours.
The carbonyl band at 1726cm-1 broadens significantly with irradiation in the films with 5 and 15%
w/w anatase, while in the film with 30% w/w anatase there is no such broadening. Also, the peak at
1477cm-1 becomes a shoulder in the film with 5% w/w anatase after six hours of irradiation, while
in the film with 15% w/w anatase this peak becomes a shoulder after 12 hours. In the film with 30%
w/w anatase the peak does not undergo any change with irradiation. It is clear that the presence of
30% w/w anatase in the Paraloid film has completely protected the film from photodegradation. The
effect of the different concentrations of anatase on the photodegradation of Paraloid film is also
shown in Figure 5.
The Paraloid films with 5, 15 and 30% w/w anatase/ rutile mixture gave films smaller protection
against photodegradation after six and 12 hours in comparison to the corresponding films with
anatase. This is shown clearly in Figure 3.
Figure 5 FTIR spectra of Paraloid B72 with 5, 15 and 30% w/w anatase after irradiation for 12
hours.
Figure 6 GPC curves of Paraloid B72 (a) before and (b) after irradiation by UV light for 12
hours.
In pure Paraloid after irradiation for 12 hours the peak at 1477cm-1 becomes a shoulder. This
change is less strong in the film with the anatase/rutile mixture and predominant in the film with
anatase. Thus anatase seems to provide greater photoprotection than the anatase/rutile mixture.
Molecular weight distribution
The molecular modification of Paraloid films occurring during photo-irradiation is shown in Figure
6, where the GPC chromatograms are reported.
Paraloid without pigment showed a strong degradation after irradiation, leading to much lower
molecular weight products. The presence of TiO2 pigments, on the contrary, protected the polymer
in the following order: 30% w/w anatase > 15% w/w anatase > 15% w/w anatase/rutile (Figure 7).
After irradiation, the
Figure 7 GPC curves of (a) Parabid B72 before irradiation and (b) Paraloid B72 with 15% w/w
anatase/rutile mixture, (c) Paraloid 872 with 15% w/w anatase and (d) Paraloid B72 with 30% w/w
anatase after irradiation for 12 hours.
Paraloid films with pigments contained a significant amount of high molecular weight molecules.
From the GPC chromatograms, the number average molecular weight (Mn) and weight average
molecular weight (Mw) of polymer samples were calculated. The results obtained are presented in
Table 2.
Solubility
Solubility is a desirable characteristic for a coating which is used for the protection of monuments.
The coating must maintain its molecular characteristics for a long time, with the possibility of being
removed by dissolution in appropriate solvents. Paraloid B72, like all acrylic coatings, is soluble in
a variety of organic solvents such as acetone, chloroform and tetrahydrofuran. Total or partial
insolubilization of acrylic coatTable 2 Molecular weights, molecular weight distribution and insoluble fraction of Paraloid
films after photo-irradiation by UV light for 12 hours
(a) Mn : number average molecular weight
(b) Mw : weight average molecular weight
(c) Mw/Mn : polydispersity index
ings has been reported as a long-term aging effect resulting from cross-linking reactions.
The effect of UV irradiation of Paraloid without and with TiO2 pigments on solubility is expressed
by the percentage insoluble fraction of Paraloid in THF, which is shown in Table 2. The partial
insolubilization of Paraloid after irradiation shows that cross-linking reactions also take place, along
with the scissions of the main chain. The results confirmed the increase in photostability when
anatase pigment was present in the Paraloid film. In the anatase/rutile mixture, however, the
percentage insoluble fraction was much higher (34%) than that of the pure Paraloid film after
irradiation (25%).
CONCLUSIONS
To begin with, it seems that the presence of titanium dioxide pigments in Paraloid B72 films
significantly improved the photostabilization of films against UV irradiation. FTIR spectra showed
that anatase provided greater photoprotection than the anatase/rutile mixture, in the following order:
30% w/w anatase > 15% w/w anatase > 15% w/w anatase/rutile. The same order was also
confirmed by the GPC measurement. The pigments most probably acted as physical UV absorbers
because they did not come in contact with atmospheric oxygen and water. Anatase with specific
surface (250m2g-1) showed better results than anatase/ rutile (65m2g-1) under the same experimental
conditions. However, this conclusion is partially opposed to that derived from the measurement of
solubility, where the pure Paraloid film after irradiation showed better results (much smaller percent
insoluble fraction) than that of the mixture with 15% w/w anatase/rutile.
ACKNOWLEDGEMENTS
This work has been realized with financial support from the General Secretary of Research and
Technology of Greece. Target project: Restoration-Conservation and Elevation of the Monuments
from the Municipal Cemetery 'Drapanos' in Argostoli, Kefallonia. The authors wish to thank F.
Zorba, chemist, for taking the FTIR spectra and D. Achillas, lecturer, for taking the GPC
chromatograms.
REFERENCES
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AUTHORS
P. SPATHIS, engineer of mines and metallurgy, diploma 1978, National Technical University of
Athens, and PhD thesis on physical chemistry and applied electrochemistry, 1982, working in the
group of Professor Th.N. Skoulikidis. Lecturer in the Department of Chemistry, Aristotle
University of Thessalomki (1984), assistant professor (1989) and associate professor (1998).
Specialized in corrosion and protection of materials. Address: Aristotle University of Thessalomki,
Department of Chemistry, Laboratory of General & Inorganic Chemical Technology, 540 06
Thessalomki, Macedonia, Greece. Email: spathis@them.auth.gr
E. KARAGIANNIDOU graduated in chemisty from the Aristotle University of Thessaloniki in 2001.
Currently attending a postgraduate programme on 'Protection, Conservation and Restoration of
Works of Art and Mechanisms' in the Polvtechnic School of the Aristotle University. Address:
Olympiados 7, Sykies, 566 26 TJtessaloniki, Greece.
A. MAGOULA, chemistry student, Aristotle University of Thessaloniki. Address: as for Spathis.
Resume — Le marbre et la pierre utilises comme materiaux de construction sont susceptibles de
subir dcs dommages dus a 1'environnement. On peut utiliser des polymeres comme supports de
pigments convenant pour la protection superficielle. La photodegradation d'une serie de films de
Paraloid B72 contenant des pigments au dioxyde de titane (TiOJ a ete etudiee sous radiations UV.
Deux sortes de dioxyde de titane, I'anatase et un melange anatase/rutile, ont ete utilisees a
differentes concentrations, 5, 15 et 30% en poids. Les changements chimiques et moleculaires
induits par {'irradiation etaient suivis par spectroscopie IRTF, chromatographie sur gel et par des
mesures de solubilite. La presence du pigment anatase ameliore de facon significative la
photostabilite des films de Paraloid.
Zusammenfassung — Marmor und andere Gesteine konnen durch Unwelteinflusse zerstort
werden. Polymere konnen als Trager van geeignetcn Schutzpigmenten fur die Oberflache der
Objekte venvendet werden. Der unter dem Einflufi van UV-Strahlung stattfindende photochemische
Abbau van Paraloid B72-Filmen unter dem EinfluJ) van Titandioxidpigmenten wurde untersucht.
Zwei Sorten van Titanoxid (TiOj, Rutil und Anatas, soune deren Mischungen ivurden in
Konzentrationen van 5, 15 und 30% (w/w) veru'endet. Die durch die Strahlung hervorgerufenen
chemischen Veranderungen wurden mil Fouriertransforminfrarotspektroskopic,
Gelpermcationschmmatographie und Loslichkeitstests untersucht. Anatas verbessert als Pigment
die Photostabilitdt van Paraloidfilmen.
Resumen — Tanto el marmol como la piedra usados como materiales de construction son
susceptibles de dano por efecto del medioambiente. Ciertos polimeras pueden ser usados, con la
inclusion de ciertos pigmentos, para la protection de la superficie de un monumento. La
fotodegradation de una serie de peliculas de Paraloid B72 conteniendo pigmentos de dioxido de
titanio (TiO7) se estudio bajo radiation ultravioleta. Se utilizaron dos tipos de T/O9) anatasa y
mezcla de anatasa/rutilo, en diferentes concentraciones: 5, 15 y 30% (peso). Los cambios
quimicos y moleculares inducidos por la irradiation UV fueron seguidos por mediciones usando
espectroscopia infrarroja transformada de Fourier, cromatografta de permeabilizacion en gel y
pruebas de solubilidad. La presencia de anatasa como pigmento mejoro sensibletnentc la
estabilidad de las peliculas de Paraloid.