New Insights into the Cleaning of Paintings

number 3 • 99
METHODS
The study of the effect of solvent mixtures on commercial
oil and alkyd paints focused on the behavior of the crosslinked
matrix upon solvation, the solubility, and leachability of
low molecular weight compounds. Paint products used were
Schmincke Norma Professional- Ölfarbe 11704, PBk 9, 37 mL
and Winsor & Newton Griffin alkyd paint 1302340, PBk 9, 37
mL at a film thickness of 300 mm. The alkyd tested is a long- oil
o- phthalic acid polyester derivative often used in oxidative drying
of alkyd paints (Ploeger, 2009). The paint films were aged
over 12 months while applying True- Lite 5500 K daylight under
window glass at 35°C–40°C. Because of the similarity of the
chemical nature of the binders in these paints it was possible to
compare selective solvation effects. The experiments were run
with six solvents exhibiting characteristic interactive properties,
n- hexane, toluene, chloroform, diethyl ether, acetone, and
ethanol (pure, Merck), as well as binary mixtures thereof at five
different molar mixing ratios, 0.15:0.85, 0.35:0.65, 0.50:0.50,
0.65:0.35, and 0.85:0.15, in correspondence to data collected
by Marcus (2002). Swelling tests were performed after Zumbühl
(2005). The swelling capacity upon immersion was used
to quantify solvation effects on the binder matrix and explore
its relation to the parameters of the solvent mixtures. The linear
solvation energy relationship based on the solvatochromic system
developed by Catalán (2001) was applied to characterize
the solvent mixtures. A PerkinElmer LAMBDA 750 UV/Vis/NIR
spectrometer was used at a spectral resolution of 1 nm to quantify
the extraction of binding media components from leachates
of alkyd paint films immersed in 25 binary ethanol mixtures for
1 and 10 minutes (maximum swelling). Extracts of 2 g of paint
sample in 50 mL of solvent were gravimetrically quantified and
characterized using Fourier transform infrared spectroscopy
(FTIR), direct temperature resolved mass spectrometry, and gas
chromatography mass spectroscopy. The FTIR measurements
were performed on a PerkinElmer System 2000 and a Bruker
HYPERION 3000/Tensor 27 at 4 cm –1 spectral resolution and
32 scans. For spectroscopic distinction of the carboxylic acid
groups from other oxidation products, samples were rolled out
on CVD diamond windows and exposed to sulfur tetrafluoride
(SF 4
) for 16 hours in a Teflon chamber (Mallégol et al., 2000).
Direct temperature- resolved mass spectrometry (DTMS) measurements
were carried out on a Thermo DSQ II quadrupole
instrument equipped with a direct probe module. The sample
was homogenized in methanol (2 mL to 1 mg of sample). Approximately
0.5 mL of the suspension (equivalent to 0.25 mg
sample) was applied to the rhenium filament, and the solvent
was allowed to evaporate. The filament was then introduced
into the ion source and resistively heated by ramping the applied
current. (Probe experimental parameters were as follows:
5 s at 0 mA, ramp 10 mA/s to 1000 mA, and 20 s at 1000 mA.
Mass spectrometer parameters were as follows: ion source temperature,
220°C; electron lens, 100 V; electron energy, –16 eV;
emission current, 45 mV; lens 1, –25 V; lens 2, –8 V; lens 3, –25
V; prefilter –12.5 V; full scan mode mass to charge ratio m/z,
42–1050; scan rate, 9700 amu/s.) Analyses were carried out in
triplicate.
RESULTS AND DISCUSSION
The solvation properties were characterized by analyzing
the swelling behavior of free paint film, expressed in volume
change DV/V 0
(Zumbühl, 2005). The absolute swelling values
are different for oil and alkyd paint. This is due to distinct densities
within the cross- linked system and variable mobility of the
polymer chains. All 75 binary solvent mixtures within the 15
experimental series exhibited nonideal properties (Figure 3).
Nonideal swelling properties of ethanol mixtures were
similarly documented by Phenix (2002). In order to qualify the
selective solvation, the parametrization of the solvent mixtures
was carried out using the Catalán (2001) solvatochromic linear
solvation energy relationship system. Results show that solvation
of oil paint systems is dominated by dispersive interactions,
whereas dipole- dipole interactions as well as specific protic and
aprotic interactions are of negligible importance. The solvation
of the oxidative functional groups is only of minor influence.
This is also true for mature oil paint films, where the absolute
swelling capacity decreases with age while the relative solvent
sensitivity remains nearly unchanged (Stolow, 1978). Cosolvation
was observed in alkyd paint systems exhibiting a very distinct
swelling sensitivity when compared to oil paint. This can
be explained through dipolar solvents, e.g., acetone, showing a
dipole- induced interaction with polarizable functional groups,
such as aromatic rings and ester groups within the phthalic acid
polyester structure of the alkyd. The nonideal swelling properties,
however, cannot be due to selective solvation because no
relation between these two factors was observed.
Nonideal Action of Solvent Mixtures
The swelling data reveal almost equivalent swelling anomalies
within oil and alkyd paints. In extremis, the swelling volume
can reach several times the ideal value, as shown in Figure 3. It
seems obvious that this effect is not influenced by the liquid- solid
interactions but is unambiguously caused by liquid- liquid interactions.
As a general rule, the larger the difference in polarity is
between the mixed solvents, the greater the observed deviation is
from the ideal behavior. In apolar mixtures this deviation from
ideal behavior is generally small. In contrast, mixtures containing
a polar solvent deviate significantly and may exhibit strong
anomalies in the swelling behavior (Figure 4).
As a consequence, all ethanol mixtures induce very strong
swelling anomalies in oil and alkyd paints, with an increase in
volume of up to 200%. This effect is particularly pronounced
with ethanol mixtures forming azeotropes, an observation that is
consistent with the findings for different polymers: here the solubility
increases if at least one of the components strongly interacts