YELLOWING OF OIL-BASED PAINTS
Jacky Mallégol, Jacques Lemaire and Jean-Luc Gardette
Summary—The cause of yellowing of oil-based paints has been investigated by analysing drying
oils simultaneously by iodometry (to determine the degree of oxidation) and by colorimetry. It was
found that yellowing of drying oils can be attributed to co-oxidation reactions of contaminants.
Yellowing level is closely related to the extent of drying and appears to be unaffected by increase in
temperature, the addition of driers, or linolenate content.
Introduction
Yellowing of drying oils used in paintings has been extensively studied because of its practical
importance. Liquid drying oils are colourless and yellowing occurs during formation of the solid oil
film by an oxidative polymerization mechanism, which is now quite fully described [1-3].
Unsaturated structures responsible for the yellowing have very high molar extinction coefficients e
and, as a consequence, very low concentrations of these products are sufficient to give a visible
yellowing. Because concentrations are low, it is rather difficult to determine the molecular
structures of yellowing compounds using various in situ spectrophotometric techniques. This lack
of information concerning molecular structures has led to various hypotheses, recently reviewed by
Kumarathasan et al. [4]. Possible causes of yellowing are summarized below.
(a) Formation of conjugated compounds resulting from decomposition of hydroperoxides.
Franks and Roberts [5] reported the existence of a direct relationship between hydroperoxide
decomposition rate and yellowing rate determined by measuring the absorbance at 350nm [6].
However, as mentioned by O'Neill et al. [7], oxidation products such as diketones or conjugated
ketones produced by hydroperoxide decomposition cannot explain the intense yellowing
observed. Yellowing of fatty esters has been associated with the formation of a-p diketo-polyene
structures. For example, conjugated diketo-trienes like
can be formed on linolenic acid chains [8] throughout oxidation, the first hydroperoxide often being
external to double bonds [9]. Elm [10] has noted that after partial curing of an oil
film, yellowing may increase because mobility between chains of radicals from hydroperoxide
decomposition reactions becomes less important. Then alkoxyl radicals, instead of recombination
reactions, could lead to particular structures with carbonyl groups.
(b) Formation of yellowing compounds resulting from condensation reactions between fatty
acid chains throughout oil drying.
Polyenes with five double bonds could also provoke an intense yellowing [11, 12]. A mechanism
accounting for the formation of polyenes in oil films, including ketone condensation, has been
proposed [4]. According to Privett et al. [13, 14], yellowing precursors are formed during the
oxidative curing step and these compounds can condense, leading to absorbing products [6].
(c) Condensation of atmospheric contaminants containing nitrogen atoms with fatty acid
chains.
Reactions with nitrogenous compounds are also mentioned in the literature as a possible
explanation for yellowing [7, 15-17]. When exposed to air, concentration of nitrogen atoms in
oils increases [18]. However, the mechanism of this incorporation is not clear.
In addition to these three hypotheses, another cause of yellowing must be considered, because naturals oils are very complex mixtures of various compounds and must be purified before use [19].
Minor constituents are not totally eliminated by refining operations such as neutralization or
bleaching. In refined oils, beside triglycerides, there remain minor quantities of undesired products
that are impurities. Moreover, chemical defects may be present in the triglyceride fatty acid chains.
For example, conjugated trienes can arise from the refining process of vegetal oils containing
linoleic acid [20]. All these undefined structures, called 'contaminants', are likely to evolve into
yellowing compounds when subjected to oxidation, and must be taken into account in an
explanation of the yellowing.
Therefore it appears clearly that the yellowing process of vegetal drying oils is a very complex phenomenon, all the more because various parameters can influence the yellowing [14, 21-23]:
temperature, humidity, radiation intensity and wavelength, irradiation time, linolenic acid content,
drier used, age and former treatments of the oil, thickness of the sample. To gain a correct
understanding of this process, we have studied the yellowing of refined linseed oil and poppyseed
oil films submitted to thermo-oxidation and photo-oxidation. This paper deals with a new approach
to determining the cause of yellowing of an oil, based on measurement of the extent of drying.
Experimental procedures
Materials
Linseed oil and poppyseed oil differ markedly in their fatty acid contents determined by proton
nuclear magnetic resonance [24]. Linseed oil contains a high proportion of linolenic acid with three
double bonds (54% linolenic. 13% linoleic. 22% oleic, 11% saturated) and poppyseed oil contains a
high proportion of linoleic acid with only two double bonds (1% linolenic, 64% linoleic, 24% oleic,
11% saturated). Saturated acids are mainly stearic and palmitic acids. Iodine values (AFNOR standard NFT 60-203) for linseed oil and poppyseed oil were 182 and 134 respectively, and acid values
(AFNOR standard NFT 60-204) were 7-9 and 6-3 respectively.
The cobalt drier used in this study was cobalt 2-ethylhexanoate.
Sample preparation
For colorimetric measurements and peroxide value (PV) determination, oil samples were dried on
rigid aluminium sheets. For the preparation of samples with cobalt drier, 0-02wt% in metal of
cobalt drier was mixed with linseed oil and immediately spread out on aluminium sheets. Many sets
of samples were made with a controlled thickness of about 30μm, for the comparative study of PV
and colour. The PV determination required as many samples as points in the curves, while the
colorimetric measurements were always performed on the same four samples and then averaged.
Two sets of linseed oil samples with thickness of 40 and 50μm were also made for an investigation
of the influence of thickness on the colour intensity.
For infrared, ultraviolet (UV) and fluorescence analyses, samples were spread out with a thickness
of about 20μm on KBr windows.
All analytical measurements were performed immediately after the thermo- or photo-oxidation
periods mentioned in the figures.
Thermo-oxidation and photo-oxidation studies
Thermal drying (thermo-oxidation) was carried out at various temperatures in ventilated and
thermo-regulated ovens in the dark. Photo-oxidation was carried out on dried samples, immediately
after curing, in a Sepap 12/24 unit (temperature 60°C). This apparatus is equipped with four 400W
medium-pressure mercury sources (Mazda MA 400) filtered by a glass envelope that eliminates
wavelengths below 300nm. A ventilating system permits control of the temperature of the exposed
samples. The temperature was regulated at 60°C for the set of experiments reported here. Exposure
periods were monitored with an internal clock. This device, which permits an artificial accelerated
photo-aging comparable with natural outdoor weathering, has been described in many papers [see
for example 25, 26].
Infrared spectra were recorded with a Nicolet 510 spectrophotometer (resolution 4cm-1, 20-scan
summation) and UV spectra were recorded on a Perkin-Elmer Lambda 5 spectrophotometer
equipped with an integrating sphere. Fluorescence spectroscopy was performed using a Hitachi
U6000 micro-spectrofluorimeter based on an Olympus BHT2 microscope. The excitation
wavelength was 395nm.
Peroxide value determination
Oxidation state or drying extent can be estimated by iodometric determination of the peroxide value
[3]. This titration is based on the reduction of peroxides with iodide and the subsequent reaction of
iodine with excess iodide (Scheme 1).
Scheme 1.
Oxidized oil sample was weighed precisely and introduced into a 50ml balloon flask with 7ml of
10:1 isopropanol/acetic acid (propan-2-ol/ethanoic acid) solution. The solution was heated to reflux
and 2ml of isopropanol saturated at room temperature with sodium iodide were added through the
condenser. After five minutes refluxing, the balloon was ice-cooled and 1ml of distilled water was
added. The I3- liberated was determined by spectrophotometry at 357nm (ε = 25000mol-1.1.cm-1), in
a 1mm quartz cell. Peroxide value (PV) was then calculated with the equation in Scheme 2.
(absorbance) 357nm
PV (mmol.kg ) = 4000 —————————
(weight) (mg)
-1
Scheme 2.
For an oil sample, evolution of the PV as a function of oxidation time is represented by the curve in
Figure 1. This is only a schematic representation showing the general pattern of behaviour for the
PV changes and no units are indicated on the axes.
Colour measurement
When peroxide value has sufficiently decreased and oil samples are sufficiently oxidized to form
solid films, colorimetric measurements were performed. These measurements were performed with
a Shimadzu UV-2101 PC spectrophotometer equipped with an integrating sphere and a D65 source
reproducing (daylight + UV), and with a standard angle of 8°. Results were an average of four
sample measurements and were given in the CIE (L*,a*,b*) system.
Results
Characterization of thermo-yellowing
Yellowing of drying oils can be observed by numerous methods. Linseed oil samples were oxidized
at 100°C and their infrared, UV-visible and fluorescence spectra were recorded. However, none of
these methods is able to determine molecular structure of compounds causing yellowing. In the
infrared spectra represented in Figure 2a, increase in the 1650-1500cm-1 region is due to unsaturated
prod-
Figure I Typical evolution curve for the peroxide value of an oxidized drying oil and representation
of the beginning of colorimetric experiments.
Figure 2 Evolution of (a) infrared, (b) UV-visible and (c) fluorescence spectra of linseed oil sample
thermo-oxidized at 100°C.
ucts that may be yellowing compounds or yellowing precursors. These unsaturated compounds are
indeed oxidation products of contaminants since no oxidation products of triglycerides are expected
to absorb
in this region [2]. In the UV spectra (Figure 2b), increase in absorption up to 400nm is characteristic
of the formation of yellow compounds, and yellow fluorescence spectra (Figure 2c) show that
fluorescence intensity also increases with oxidation time.
Indeed, yellowness of oil films is due both to the additional effects of absorbing compounds in the
blue region and fluorescent compounds in the yellowing region. Yellowing measured by
colorimetry therefore results from absorption yellowing and fluorescence yellowing. Formation of
absorbing products and fluorescent products occurs simultaneously. This may explain why the
fluorescence phenomenon, previously described by de la Rie [27], appears to be closely related to
absorption increase in UV-spectra [28].
Colorimetric analysis of the yellowing phenomenon
Effect of drying temperature
We have studied by colorimetry the yellowing of linseed oil films during curing at different
temperatures. As an example Table 1 shows the evolution of the three factors L*, a* and b*, in a
colorimetric measurement of linseed oil oxidized at 80°C, as a function of the drying time. For each
colorimetric measurement, the peroxide value of the sample was also determined by iodometry.
L* and a* factors vary with drying time and drying extent (decrease in PV), but evolution of the b*
factor (shift to yellow on the yellow-blue chromatic axis) is continuous and sufficient to describe
the increase in yellowing. In this study, we have only used the b* factor as a measure of the
yellowing degree. Variations of b* factor and PV as a function of oxidation time are represented in
Figure 3a. They evolve in opposite ways and Figure 3b represents the plots PV = f(t) and 1/b* =
f(t).
It appears that the two plots in Figure 3b are identical. This indicates that, for linseed oil oxidized at
80°C, yellowing degree (factor b*) is closely related to the drying extent (PV). For four other
temperatures (25, 40, 60 and 120°C) plots PV = f(t) and 1/b* = f(t) also coincide. Plots for the 25°C
oxidation are presented in Figure 4, as a second example.
For all the temperatures investigated, these plots evidence a direct relationship between yellowing
(increase in factor b*) and drying extent (decrease in peroxide value), which can be explained by
co-oxidation reactions of contaminants present in oils, even if it is often neglected in the literature.
Peroxide decomposition results in two radicals that lead to a secondary initiation of oxidation reactions. Co-oxidation of oil contaminants, present in low concentrations, arises by a radical
mechanism in a reaction parallel to the normal oxidation reaction (Scheme 3).
Scheme 3.
Table 1 Evolution of the colorimetric parameters L*, a*, b* and peroxide value as a function of
linseed oil drying time at 80° C
Figure 3 (a) Evolution of the peroxide value (□) and factor b* (●) as a function of time for linseed
oil oxidized at 80° C. (b) Comparison of the plots PV = f(t) and 1/b* = f(t) for linseed oil oxidized
at 80°C.
Figure 4 (a) Evolution of the peroxide value and factor b* (●) as a function of time for linseed oil
oxidized at 25°C. (b) Comparison of the plots PV = f(t) and 1/b* = f(t) for linseed oil oxidized at
25°C.
The yellowing route can be justified by different reactivities for these two oxidation reactions. At
the beginning of the oxidation, fatty acid chains are polyunsaturated and are easily oxidizable.
Contaminants, on the contrary, are much less oxidizable and the first drying step does not produce
yellow compounds. But when drying extent increases, there are no more polyunsaturated chains and
reactivity of monounsaturated chains is about twenty-fold lower [1]. Then, from a certain drying
extent, contaminant oxidation becomes competitive and yellowing appears. As this reaction
depends on the radical concentration existing in oil films, yellowing rate is related to decrease in
peroxide value. Another argument in favour of the crucial role played by contaminants in the
yellowing phenomenon will be given further on in the text with the colorimetric study of the
yellowing/bleaching alternation in thermo-oxidation and photo-oxidation.
Effect of cobalt drier
Because excessive yellowing of dried oil films is often associated with drier use [4, 22], we have
studied the yellowing of linseed oil with cobalt drier (0-02% in metal) at temperatures of 25, 40. 60
and 80°C. Figure 5a represents the plots PV = f(t) and b* = f(t), and Figure 5b the plots PV = f(t)
and 1/b* = f(t), for linseed oil with cobalt drier during oxidation at 80°C. Again, a direct
relationship between b* and PV is apparent. Therefore, drier seems to have no effect on yellowing.
The more important yellowing reported in the literature when a drier is used is simply related to the
fact that driers favour hydroperoxide decomposition and indirectly induce the oxidation of
contaminants.
The influence of temperature and drying agent has been examined in various conditions, where a
direct relationship between b* and PV has always
Figure 5 (a) Evolution of the peroxide value and factor b* as a function of time for the mixture (linseed oil + cobalt drier) oxidized at 80° C. (b) Comparison of the plots PV = f(t) and 1/b* = fit) for
the mixture (linseed oil + cobalt drier) oxidized at 80° C.
been found. The b* values corresponding to PV = 200, 300 and 400mmol.kg-1 have been brought
together in Table 2, with the aim of demonstrating a more general relationship between b* and PV.
The b* values should be compared for the same PV in linseed oil and linseed oil with cobalt drier.
For temperatures higher than 60°C, b* values are dependent neither on the presence of the cobalt
drying agent nor on the temperature. (The yellowing appears to be enhanced by temperature
increase or drier presence, but the intrinsic yellowing tendency remains unchanged.) This has also
been shown for PV lower than 100mmol.kg-1 (not mentioned in the Table). But b* values are
manifestly different for temperatures of 40 and 25°C (identified in bold). Values at low
temperatures are always superior to those at temperatures >60°C. These results may be explained by
considering not only the decrease in PV but also kinetics of peroxide decomposition.
Hydroperoxides can decompose by homolytic scission of the O—O bond into chain alkoxyl radical
and hydroxyl radical; and dialkylperoxides can decompose into two alkoxyl radicals (Scheme 4).
Scheme 4.
It should be noted that the yellowing of oil film begins only after an important decrease in the residual PV (around PV <500mmol.kg'1)- At this time, some important drying has already occurred and
Table 2 Factor b* values for peroxide values equal to 200, 300 and 400mmol.kg-1 for pure
linseed oil samples and mixtures of linseed oil + cobalt drier, oxidized at different temperatures
the film is cross-linked. At temperatures up to 60°C, thermal peroxide decomposition is quite
important. Then radical concentration in the film is relatively elevated and radical recombination
between chains or radical propagation on another chain are highly probable, all the more in that
high temperature favours mobility of chains. And there is also water elimination by a cage reaction
that is an important route for disappearance of radicals [2]. However, at temperatures below 40°C
the radicals resulting from peroxide decomposition have lower concentration and lower mobility.
Then the reaction with another fatty acid chain is less probable than the reaction with a contaminant
molecule, which can be neighbouring. Moreover, it has been shown [3] that below 40°C, homolytic
decomposition of peroxides is competing with another decom-
Figure 6 Comparison of factor b* evolution plots as a function of peroxide value, for linseed oil
and poppyseed oil oxidized at 60 and 120° C.
position mechanism that may start by hydrogen abstraction on the tertiary carbon (Scheme 4). The
radical formed is also a chain radical with low mobility. Therefore, at low temperatures, the oxidation of contaminants increases and, for a given PV, yellowing is increased compared to
temperatures higher than 60°C.
Effect of unsaturation degree
Another statement found in the literature deals with the degree of unsaturation of oils. It is said that
yellowing increases with unsaturation [14, 29]. To verify this hypothesis we have studied yellowing
of poppyseed oil at 60 and 120°C. For poppyseed oil, we also found a direct relationship between
PV and b*. Linseed oil contains 54% of linolenic acid with three double bonds while poppyseed oil
contains only 1%. We have compared yellowing of linseed oil and poppyseed oil by representing
the evolution of the b* factor as a function of the peroxide value, at 60 and 120°C (Figure 6).
It appears that the yellowing tendency is more marked for linseed oil. Of course, this difference may
be attributed to the greater tendency of linolenic acid chains to produce more unsaturated structures
and compounds that are stronger chromophores. However, when a significant degree of drying is
reached (PV = 30mmol.kg-1 at t20°C), the magnitude of yellowing of poppyseed oil and linseed oil
appears to be of the same order. O'Neill [16] has noted that linoleic triglycerides also yellow when
oils are oxidized under severe conditions. So, the more important yellowing of linseed oil compared
to poppyseed oil when drying is not complete (PV > 200mmol.kg-1) is probably due to the higher
purity of poppyseed oil, which contains fewer contaminants or else contaminants with lower
yellowing power.
Study of the yellowing/bleaching alternation of oil films subjected to thermo-oxidation/photooxidation cycles
Another important aspect of the yellowing is its 'reversibility', as it is called by Rakoff et al. [30]. In
photo-oxidation, yellow products in oil films are decomposed rapidly [30]. But when samples are
replaced in thermo-oxidation at 100°C, yellow coloration reappears. Notable fluctuations can be
observed during cycles alternating thermo-oxidation at 100°C and photo-oxidation in the Sepap
12/24 unit for a linseed oil sample on a KBr window (Figure 7). However, this alternation of
yellowing and bleaching is not a truly reversible reaction, in the sense that non-oxidized molecules
in the first cycle can be converted into yellowing products in the following cycles [16]. It can be
noted that disap-
Figure 7 Evolution of UV-visible spectra of linseed oil sample submitted to successive thermooxidations at 100°C and photo-oxidations. (1) 200h at 100°C, (2) +24h in Sepap, (3) +1200h at
100°C. (4) +24h in Sepap, (5) +1200h at 100°C, (6) +24h in Sepap.
pearance of absorbing products up to 400nm is very fast (24-hour photo-oxidation).
Residual double bonds in dried oil film, which could be implicated in unsaturated structures
responsible for yellowing, disappear quickly under photo-oxidative conditions [31]. It is then
probable that unsaturated products on oxidized chains are decomposed in photo-oxidation and the
yellowing increase in subsequent thermo-oxidation cycles is only due to contaminants. Because thin
films are more prone to bleaching than thick films when irradiated, we have studied films with three
different thicknesses. Oil samples were oxidized on aluminium sheets with a thickness of 30. 40 and
50um and measurements of yellowing by colorimetry were performed. Figure 8 represents
evolution of the b* factor as a function of time in thermo-oxidation/photo-oxidation cycles for the
three thicknesses.
In Figure 8, influence of sample thickness is clearly apparent. For the 50um sample, yellowing
becomes more important in thermo-oxidation but its value stays higher after photo-oxidation, too.
With reference to Figure 7, an important point should be remarked. When absorbing products are
decomposed in a 24-hour photo-oxidation, total yellowing decreases slightly within this duration. It
means that the contribution to yellowing of absorbing products disappears in photo-oxidation, while
the contribution of fluorescent products is partly or totally preserved, even after the 300-hour photooxidation. We have already noted that fluorescence products are stable under irradiation in vacua
(photolysis experiment) [31]. Residual yellowing after photo-oxidation can therefore be explained
by the fact that the oil network becomes more cross-linked
Figure 8 Evolution of factor b*, representing yellowing, in the course of three cycles (thermooxidation at 120°C/photo-oxidation in Sepap 12/24 unit), for three linseed oil samples with
thicknesses of 30, 40 and 50μm. (A) 400h thermo-oxidation, (B) +300h photo-oxidation, (C) +400h
thermo-oxidation, (D) 600h photo-oxidation, (E) +500h thermo-oxidation, (F) +600h photooxidation.
and less permeable to oxygen. Then only weak concentrations of fluorescent products are photooxidized and fluorescence yellowing maintains a quite significant level.
During the second thermo-oxidation, increase in yellowing can be attributed to the oxidation of
internal contaminants that had not yet reacted, and possibly to the formation of new compounds
resulting from decomposition products (condensation reactions proposed in the literature). The
second photo-oxidation leads to less marked bleaching, probably because of lower oxygen
diffusion.
As and when cycles arise, yellowing in thermo-oxidation becomes less and less important for the
three samples, but in photo-oxidation it keeps a higher and higher value. So evolutions seem to
show a convergence toward a degree of yellowing corresponding to a state with photo-stable
yellowing products and without any formation of new yellowing products. It may be noted that
photo-oxidation provokes an important chemical evolution. We have shown that dried oil films are
very-sensitive to photo-oxidation and that the decrease in yellowing is accompanied by an
important network degradation and loss of matter [31].
Conclusion
Our results have shown that yellowing is closely related to the extent of drying. In our opinion, yellowing of drying oils is mainly due to contami-
nants. At temperatures up to 60°C, parameters increasing the oxidation rate—such as higher temperature or presence of a drier—only increase the yellowing rate but have no influence on the
yellowing tendency. For temperatures below 40°C, when reactions of fatty acid chains are slowed
down, reactions with contaminants appear to be favoured and yellowing tendency is higher.
Differences in degrees of yellowing for different oils may be attributed to various concentrations of
contaminants or various sorts of contaminants. Comparative studies of yellowing are then very difficult to interpret, even with comparable drying extents. For a given oil, yellowing will increase
until there are no more contaminants or until the complete drying of the oil sample is achieved, with
no more radicals formed in the film. Since yellowing is also significant in poppyseed oil containing
less than 1% of linolenic acid, the role generally attributed to this polyunsaturated acid in the formation of yellowing products appears to be unfounded.
Solutions to avoid yellowing of oil-based paints appear to be very difficult to achieve. Only
extremely purified oils or synthetic compounds may lead to colourless films, and an elevated drying
temperature would be required to reduce interactions with contaminants.
However, it must be noted that yellowing, although unaesthetic, is not a significant criterion of
physical degradation because it involves only weak concentrations of contaminants and leaves the
network unaffected. Even more, because it is a measurement of the extent of drying, it may be
considered as a proof of the hardness of the film that is completely dried. Yellowing must therefore
be considered as an unavoidable characteristic of drying oils and this must be kept in mind by users.
Suppliers
Sepap 12/24 unit: Atlas Material Testing Technology, Le Mesnil Armelot. France.
Nicolet 510 spectrophotometer: Madison, Wisconsin, USA.
Perkin-Elmer
Lambda 5 spectrophotometer: Courtaboeuf, France.
Shimadzu UV-2101 PC: Columbia, Maryland, USA.
Linseed oil and poppyseed oil were supplied by Pebeo (Gemenos, France). Cobalt 2-ethylhexanoate was supplied by Aldrich (Gillingham, UK). Isopropanol (99-5%, HPLC grade) was supplied
by Sigma-Aldrich, acetic acid (99-100% for synthesis) by Merck (Schuchardt. Germany) and
sodium iodide (99-5%) by Prolabo (Paris, France).
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29 RAKOFF. H., THOMAS. F.L., and GAST, L.E., "Yellowing and other film properties of linseedderived paints influenced by linolenate content', Journal of Coatings Technology 48 (1976) 55-57.
30 RAKOFF, H., THOMAS, F.L., and GAST, L.E., "Reversibility of yellowing phenomenon in linseedbased paints', Journal of Coatings Technology 51 (1979) 25-28.
31 MALLEGOL. J.. GARDETTE, J.-L., and LEMAIRE, J.. 'Long-term behavior of oil-based varnishes
and paints. 3. Photo- and thermo-oxi-dation of cured linseed oil'. Journal of the American Oil
Chemists' Society 77 (2000) 257-263.
Authors
JACKY MALLEGOL received
his chemical engineering diploma from the Ecole Nationale Superieure
de Chimie de Clermont-Ferrand in 1995, with a specialty in organic materials. He received his PhD
in physical chemistry in 1999 from the Blaise Pascal University of Clermont-Ferrand, on the curing
and long-term behaviour of drying oils used in paints. Since January 2000. he has had a
postdoctoral position at the Institute for Chemical Process and Environmental Technology, Ottawa
(Canada). Address: Centre National d'Evaluation de Photo-protection, Ensemble Scientifique des
Ce:eaux, 63177 Aubiere Cedex, France.
JACQUES LEMAIRE is
professor of chemistry at the Blaise Pascal University in Clermont-Ferrand
(France) where he was responsible for the establishment of the Laboratoire de Photochimie, which
carries out basic research in the areas of molecular and macromolecular photochemistry. In 1986, he
founded the Centre National d'Evaluation de Photoprotection, a research centre which works in
close cooperation with the Blaise Pascal University, industrial companies and art centres. The
primary focus of the centre is the analysis of the long-term behaviour of organic materials based on
their aging as a result of light exposure. Address: as for Mallegol.
JEAN-LUC GARDETTE [author
to whom correspondence should be addressed] joined the Laboratoire
de Photochimie Moleculaire et Macromoleculaire in 1974. where he works on fundamental aspects
of photo-degradation and photo-stabilization of polymers. Other areas of interest include the
applications of vibrational spectroscopy (mainly FTIR) to the field of polymers. In 1992, he was
promoted to Directeur de Recherche au CNRS and in 1993. Director of the Laboratoire de
Photochimie. In 1998. he also became professor of physical chemistry at the University Blaise
Pascal. Address: Laboratoire de Photochimie Moleculaire et Macromoleculaire, UMR CNRS 6505,
Ensemble Scientifique des Cezeaux, 63177 Aubiere Cedex, France.
Resume—On a recherche la cause du jaunissement des peintures a Vhuile en analysant des huiles
siccatives simultanement par iodometrie (qui permet de determiner le degre d'oxydation) et par
colorimetrie. On a trouve que le jaunissement des huiles siccatives peut etre attribue a des
reactions de co-oxydation de contaminants. Le niveau de jaunissement est etroitement lie au degre
de sechage et ne semble pas etre affecte par I'elevation de la temperature, I'addition de siccatifs ou
le contenu en linoleates.
Zusammenfassung—Die Ursache fur das Vergilben van olhahigen Malschichten wurde durch
Analyse der trocknenden Ol durch Iodometrie (zur Bestimmung des Oxidationsgrades) bei
gleichzeitiger Farbmessung untersucht. Es konnte gezeigt werden, dafi die Vergilbung trocknender
Ole der Oxidation von Verunreinigungen zugeschrieben werden kann. Der Grad der Vergilbung ist
dabei eng mil dem Ausmafi der Trocknung verbunden, scheint aber unabhangig von
Temperaturerhohung, der Anvvesenheit von Trocknungsbeschleunigern und dem Gehalt an Linolat
zu sein.
Resumen—La causa del amarilleamiento en la pinturas de tipo oleoso ha sido investigada por
media del andli-sis de los aceites secativos en combination con iodometria (para determinar el
grado de oxidation) y de co-lorimetria. Se determine que el amarilleamiento de los aceites
secativos puede atribuirse a las reacciones de co-oxidacion de los contaminantes. El nivel de
amarilleamiento esta intimamente relacionado con el nivel de secado y parece no estar afectado
por el incremento de la temperature, la adicion de secativos o el contenido de linolenato.