Microchemical Journal 124 (2016) 949–961

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Microchemical Journal

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Accelerated UV ageing studies of acrylic, alkyd, and polyvinyl acetatepaints: Influence of inorganic pigments

Valentina Pintus a,1, Shuya Wei b, Manfred Schreiner a

a Institute of Science and Technology in Art, Academy of Fine Arts, Schillerplatz 3, A-1010 Vienna, Austriab Institute of Historical Metallurgy and Materials, University of Science and Technology Beijing, 30 Xueyuan Road, 100083 Beijing, China

E-mail address: v.pintus@akbild.ac.at (V. Pintus).1 Tel.: +43 1 58816 8680; fax: +43 1 58816 8699.

http://dx.doi.org/10.1016/j.microc.2015.07.0090026-265X/© 2015 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 26 March 2015Received in revised form 25 June 2015Accepted 9 July 2015Available online 21 July 2015

Keywords:Py-GC/MSFTIR-ATRColour measurementsAcrylicAlkydPolyvinyl acetate

The stability of two types of acrylic bindingmedia, alkyd and polyvinyl acetate (PVAc), fourwidely used syntheticbinders inmodern and contemporary art, to UV light also including theUV-B range (315–280 nm,middle UV) forsimulating sunlight outdoor conditions was studied and compared by double-shot and single-shot Py-GC/MS,FTIR-ATR, and colour measurements. Thermally assisted hydrolysis and methylation (THM-GC/MS) analyseswere used for the alkyd. Additionally, the influence of inorganic pigments on the photo-oxidative stability ofthe binding media was also considered. For this purpose, the binders in their pure form as well as mixed witheight different inorganic pigments (titaniumwhite—anatase and rutile, cadmium yellow, cadmium red, hydratedchromium oxide green, ultramarine blue, raw umber Cyprus, and ivory black) were exposed to the acceleratingartificial UV ageing for different periods of time and analysed before and after UV exposure. After UV ageing, thedouble-shot Py-GC/MS detected the effect of photo-oxidative processes of the binders in a much more detailedway than by the single shot. For instance, photo-oxidation of the acrylics resulted in a production of oligomerseven during the thermal desorption step of double-shot Py-GC/MS and in a decrease of the EA and nBA mainmonomers in the pyrolysis second step. Contrarily, the aged alkyd samples were mostly characterised by a de-crease of unsaturated fatty acids and especially by the increase of free ortho-phtalic acid detected by double-shot Py-GC/MS and THM-GC/MS, which is also reflected in the FTIR-ATR results. On the other hand, an increaseof free acetic acid was observed for the aged polyvinyl acetate by double-shot Py-GC/MS. Colour measurementsrecorded a greater sensitivity of the alkyd paints, shown by to the bigger shift of L*, a*, and b* coordinates andchange of E* values. Of additional interest was the higher sensitivity to the UV light for synthetic bindingmedia in combination with pigments.

© 2015 Elsevier B.V. All rights reserved.

1. Introduction

Currently, the bindingmedia of most paints consist of synthetic ma-terials such as acrylic, alkyd, and polyvinyl acetates, resulting in a widerange of different types with properties, and as a consequence, the ma-jority of modern and contemporary art is composed of synthetic mate-rials. Therefore, the investigation of the factors influencing theirageing behaviour is becoming more and more important in heritagescience. The stability of these binders to UV light in outdoor conditionsis particularly important. UV ageing studies of synthetic paint materialshave been carried out in detail in the last 20 years [1–3], mainly consid-ering indoor conditions that simulate parameters relevant for art ob-jects housed in museums, such as wavelengths between 400 and315 nm (UV-A, near UV). Acrylic binders based on an emulsion andformed by the four basic components, water, monomer, initiator, andsurfactant, are prone to photo-oxidation mostly by chain scission reac-tions and cross-linking. The chain scissions tended to prevail over the

cross-linking reactions when the alkyl side groups are short [4].On the other hand, the photo-oxidation of alkyds is similar to thatof oil paints because of their similar chemical composition basedon the addition of monobasic fatty acids into the polyesterconformation—formed by a polyhydric alcohol and a polybasic carbox-ylic acid. Auto-oxidation of the alkyd can continue with excessive de-gree of cross-linking producing a very stiff and brittle material, chainscission with prevalent β-scissions, loss of volatile products such as al-dehydes, alcohols and carboxylic acids and fading (yellowing) [5]. Thefragments formed by the β-scission reactions can be converted intofree, low molecular weight compounds that can stay in the paint filmor can remain in thenetwork, through other cross-links along the chainsor can be lost by evaporation or by solvent treatment [6]. The photo-oxidative degradation of polyvinyl acetates (PVAc), which are formedduring the polymerisation of vinyl acetate, follows similar mechanismsteps as those of acrylic polymers, which through chain scission andcross-linking reactions produces a series of reactive intermediates andradicals [7].

In contrast to studies based on the photo-oxidation of these synthet-ic materials induced by the UV light in indoor conditions, the influence

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of UV-B light (315–280 nm, middle UV) on the stability/degradation ofmaterials in artworks exposed to outdoor conditions has been inves-tigated only rarely. UV-B radiation may cause discolouration, cracks,and other damage to synthetic materials while the UV-A (400–315 nm,near UV) is usually considered less harmful to polymers. UV-C (280–100 nm, far UV) which could cause chain scissions and/or cross-linking of the chemical structure of the polymers is absorbed by theatmosphere. Furthermore, volatile compounds, which can be formedduring photo-oxidative reactions and/or any changes, are likewise rare-ly studied by Py-GC/MS, especially in the double-shot mode [8]. Thistechnique represents a useful and powerful method for the identifica-tion and characterisation of volatile compounds, polymeric materials,and additives [8].

Another issue is that the photo-oxidative stability of syntheticbinders can be influenced by the addition of pigments into the system.They may either have a protective effect by absorbing and/or screeningtheUV light or theymaybephoto-active and therefore catalyse or accel-erate the photo-degradation of the polymer [9]. The mechanism bywhich pigments may act as photo-sensitisers is not well understood,and detailed data on the influence of inorganic pigments on the stabilityof synthetic paint films are still lacking. Additionally, synthetic paintsare characterised by other components that can also contribute signifi-cantly to their degradation under UV. Thus UV ageing studies of puresynthetic binding media does not accurately reflect the UV ageing ofsynthetic paints and the exact context of the degradation still remainsunclear inmany cases. Therefore, more studies concerning the influenceof inorganic pigments on synthetic binding media are desirable.

Partially based on previous studies [10–12], the aim of thiswork is toinvestigate the stability when exposed to UV light including the UV-Brange for simulating sunlight outdoor conditions of four of the mostwidely used bindingmedia inmodern and contemporary art, two differ-ent types acrylic, alkyd, and polyvinyl acetate (PVAc), as well as the ef-fect of different inorganic pigments on their chemical behaviour. Forthis purpose, two types of acrylic binding media, alkyd and polyvinylacetate (PVAc) both pure and also mixed in the laboratory with eightdifferent inorganic pigments (titanium white—anatase and rutile,cadmium yellow, cadmium red, hydrated chromium oxide green, ultra-marine blue, raw umber Cyprus, and ivory black) were analysed beforeand after UV exposure by the double-shot of Py-GC/MS, single-shot ofPy-GC/MS while thermally assisted hydrolysis and methylation (THM-GC/MS) analyses were used for the alkyd, FTIR-ATR, and colour mea-surements. A comparison between the double-shot and single-shottechniques of Py-GC/MS for their application for UV ageing studies ofmodern paint materials was also done.

2. Experimental

2.1. Sample preparation

The investigated binders andmock-up samples are listed in Tables 1and 2, respectively. Approximately 60 mg of two pure acrylic bindingmedia (Plextol® D498 and Primal® AC33 purchased from KremerPigmente GmbH & Co. KG, Germany) (Table 1) were cast separatelyon glass plates, which produced a dried film thickness in a range of

Table 1List of the binders investigated and their chemical and technical properties.

Binders

Commercial name Company Type of binder

Plextol® D498 Kremer® Pigmente GmbH & Co. KG, Germany Acryl

Primal® AC33 Kremer® Pigmente GmbH & Co. KG, Germany Acryl

Medium 4 Lukas®, Dr. Fr. Schoenfeld GmbH & Co., Germany AlkydMowilith® 50 Kremer® Pigmente GmbH & Co. KG, Germany PVAc

10–20 μm. Additionally, different mock-ups of pure Plextol® D498 andPrimal® AC33 mixed with inorganic pigments (Table 2) such astitanium white (anatase and rutile), cadmium yellow, cadmium red,hydrated chromium oxide green, ultramarine blue, raw umber Cyprus,and ivory black were prepared. Each mock-up was made by mixingwith a paint brush the binder and the pigment in a mixing ratio ofabout 3:1. Once a proper paste consistency was obtained, the paintwas cast on glass slides. The dried film thickness of these paint sampleswas approximately in the range of 30–40 μm. In total, 12 identical spec-imens for each pure acrylic bindingmedium and three for each pigmentmixed with an acrylic emulsion were made in order to obtain sufficientsamples for the tests in the UV chamber. After drying the samples for24 hours at room temperature, the glass plates were arranged in theUV chamber, except for unaged reference samples.

The alkyd Medium 4 (Lukas®, Dr. Fr. Schoenfeld GmbH & Co.,Germany) (Table 1), was cast on three glass plates, producing a driedfilm thickness in a range of 10–20 μm. Alike for the acrylics, the alkydoil binding medium was mixed with the selected inorganic pigmentsand consequently cast onto glass slides (Table 2). In order to obtainenough samples for the consequent exposure in the UV chamber,three identical samples for each pigment mixed with the alkyd bindingmedium were made. Similar to the acrylic samples, pure alkyd bindingmedium and their mixtures with pigments were left dry for 24 hoursat room temperature.

Polyvinyl acetate Mowilith® 50 (Kremer Pigmente GmbH & Co. KG,Germany) was chosen (Table 1) for making specimens, which wasdissolved in acetone, cast on glass slides and then allowed to dry atambient conditions (average thickness of the paint film was about10–40 μm). The selected inorganic pigments used for the acrylicmock-ups were mixed with the polyvinyl acetate Mowilith® 50 to pre-pare mock-up samples on glass slides (Table 2) and left to dry at roomtemperature.

The inorganic pigments used for all mock-ups are products ofKremer except the white pigments, which are part of the material col-lection of the ISTA (Institute for Science and Technology in Art) at theAcademy of Fine Arts Vienna.

2.2. UV exposure

UV exposure of the samples was carried out in a UVACUBE SOL 2/400F UV chamber, produced by Dr. Hönle GmbH UV-Technology,Germany. The UV light radiation source was supplied by a 910 W/m2

Xenon arc solar simulator with an incorporated H2 filter, which pro-vides radiation with wavelengths between 295 and 3000 nm thus sim-ulating sunlight outdoor conditions. The chamber temperature was48.8 °C. No control of the relative humidity (RH) was possible in theUV exposure chamber used, therefore the RH varied between 30% and35% depending on the RH of the ambient atmosphere. The acceleratedUV exposure of the acrylic and alkyd samples was carried out for 31and 83 days, respectively, while the polyvinyl acetates were UV agedfor 60 days. According to the ASTM 2565 – 99 standard [13], theexposure time of the specimens under UV light was established from along-term evaluation and control of the material. This evaluationwas periodically carried out under an optical microscope to observe

Composition Product number

Acqueous dispersion of a thermoplastic acrylic polymer based on:poly(n-butyl acrylate / methyl methacrylate), p(nBA/MMA)

76000

Acqueous dispersion of a thermoplastic acrylic polymer based on:poly(ethyl acrylate / methyl methacrylate), p(EA/MMA)

75200

Solution of alkyd resin in mineral spirit 2224Polyvinyl acetate in grainy state 67040

Table 2List of the mock-up samples investigated and their chemical and technical properties.

Mock–up samples

ColorPigment

classical name

Binder Chemicalcomposition of

pigment

Color index:generic name

Constitutionnumber ofpigment Acryl:

Plextol®D498

Acryl:Primal®

AC33

Alkyd:Medium 4

PVAc:Mowilith®50

WhiteTitanium white

anatase TiO2 PW 6 69

WhiteTitanium white

rutile TiO2 PW 6 54

Yellow Cadmium yellow CdS PY 37 21120

Red Cadmium red CdS, xCdSe PR 108 21060

Green

Hydratedchromium oxide

greenCr2O3•H2O PG 18 44250

Blue Ultramarine blue Na8Al6Si6O24•Sx PB 29 45010

BrownRaw umber

Cyprus Fe2O3•H2O, MnO2 PBr 8 40610

Black Bone (ivory) black C, Ca3(PO4)2, CaCO3 PBk 9 47150

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any change in the morphology of the surface, by Fourier transform at-tenuated total reflection (FTIR-ATR) analysis (Section 3.3) and colourmeasurements (Section 3.4).

By using the Hönle UV-Meter (Dr. Hönle GmbH UV-Technology,Germany), it was observed that during the UV ageing of the alkyd sam-ples, the UV radiation intensity of the xenon lamp reached a value of ap-proximately 177 W/m2, thus exposing the samples to a more gentleageing than the acrylic and polyvinyl acetate specimens. Unfortunately,the extended use of UV lamps results in a gradual decrease of the inten-sity over time, which has to be taken into account for the UV ageingstudies.

2.3. Pyrolysis gas chromatography mass spectrometry (Py-GC/MS)

For the UV-B ageing studies, all samples were analysed before andafter UV exposure with double-shot and single-shot Py-GC/MS.Double-shot Py-GC/MS is based on a two-step analysis: 1) Thermal de-sorption of the samples at lower temperature to detect volatile com-pounds and 2) pyrolysis of the same sample as second step.

Between 80 and 300 μg of the samples were scraped from the glassplates with a scalpel and put in a sample cup (ECO-CUP Frontier Lab,Japan) for analysis. Unlike acrylic and the polyvinyl acetate paints, about100 μg of alkyd samples were treated with 2 μL tetramethylammoniumhydroxide (TMAH) reagent (25 wt% aqueous solution of TMAH, Sigma-Aldrich, USA) in the sample cups in order to perform thermally assistedhydrolysis and methylation (THM-GC/MS) analysis. Py-GC/MS andTHM-GC/MS analyses were performed with a PY-2020iD (Frontier Lab,Japan) pyrolyzer unit combined with a GCMS-QP2010 Plus (Shimadzu,Japan). For the analyses of the pure acrylic binding media as wellas those mixed with pigments, the GC/MS unit was equipped with acapillary column SLB-5ms SUPELCO, USA (30 m length × 0.25 mminternal diameter × 0.25 μm film thickness) using bonded andhighly cross-linked 5% diphenyl / 95% dimethyl siloxane. Due to thechange of the column after the initial measurements, the Ultra Alloy-5

MS (5% diphenyl / 95% dimethyl siloxane) capillary column with a0.25 mm internal diameter, 0.25 μm film thickness, and 30 m length(Frontier Laboratories, Japan) was used for the analyses of the alkydsand polyvinyl acetates afterwards. NIST 05 and NIST 05s Library ofMass Spectra were used for the identification of the compounds. TheGC column temperature conditions used for the acrylics and polyvinylacetates were as follows: initial temperature 40 °C, held for 5 minfollowed by a temperature increase of 10 °C/min to 292 °C. For thealkyds, the oven initial temperature was set at 40 °C for 5 min, andthen followed with a gradient of 6 °C/min up to 280 °C for 10 min.The heliumgas flowwas set at 1mL/min andmass spectrawere record-ed under electron impact ionisation at 70 eV. For the single-shot analy-ses, the pyrolysis temperature was set at 600 °C and held for 12 s.

The double-shot parameters were as follows: For the thermal de-sorption of the pure acrylic binding media and their mixtures with pig-ments, the temperaturewas set at 50 °C, held for 2min and increased by20 °C/min to 300 °C and held there for 2 min while for the alkyds thetemperature was set at 50 °C and increased by 20 °C/min to 250 °C. Ad-ditionally, for the polyvinyl acetates, the temperature was set at 100 °C,held for 2 min and increased by 20 °C/min to 250 °C and held there for2 min. The second-step pyrolysis was carried out for all samples at600 °C and held for 12 s.

2.4. Fourier transform infrared spectroscopy-attenuated total reflectance(FTIR-ATR)

FTIR-ATR analyses were performed with an Alpha FT-IR PlatinumATR instrument (Bruker Optics, Germany) equipped with a deuteratedtriglicine sulphate detector (DTGS) andwith a diamond crystal. Spectrawere acquired in a spectral range between 4000 and 370 cm−1

performing 64 scans at 4 cm−1 resolution. The resulting spectra werecollected and evaluated with the spectrum software OPUS® of BrukerOptics, Germany.

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The infrared absorptions bands obtained for the investigated pig-ments were identified by comparing the acquired spectrum of the pig-ment sample with the reference spectrum of the IRUG (InfraredRaman Users Group) database.

2.5. Colour measurements

A SPM50 (Gretag-Macbeth AG, Switzerland) instrumentwas used toobtain spectra in the visible range. Reflectancemeasurementswere car-ried out using a D65 lamp in the range of 380–730 nm. Reflection wasmeasured relative to the white standard of the instrument and a 10°Standard Observer was used. For the UV ageing studies, the total colourvalues (ΔE*) are obtained according to the Commission Internationalede l’Eclairage (CIE) 2000.

3. Results and discussion

The comparison between the results obtained fromunaged and agedsamples reveals that the effects of the photo-oxidative reactions weremost determinable after 83days of UVexposure. Therefore, the compar-ison between the results acquired was made between the unaged and83-day aged samples.

3.1. Impact of UV ageing to the paints

The visual appearance of paint colours is a fundamental characteris-tic for the evaluation of their state of preservation. Any variation of thematerial properties to the influence of UV light can correspond to achange of colour, opacity, and brittleness. Thus, a systematic investiga-tion of these chemical changes is an important point of study for theconservation and restoration of paints.

After the ageing periods, some of the samples showed a slight colourchange and different opacity. The colour and opacity of blue mock-upswere the most affected by UV light. By scraping samples from theaged films for analysis, itwas observed that they had becomemore frag-ile and brittle compared to unaged samples. These changes in theirmac-roscopic properties can be considered characteristic for UV ageing andcan be related to their chemical changes described in the next threeSections 3.2, 3.3, and 3.4.

3.2. Py-GC/MS analysis of unaged and aged samples

In order to investigate the photo-oxidative deterioration of the sam-ples when exposed to UV light, each of the unaged and aged acrylic andpolyvinyl acetate (PVAc) binder sample types were analysed by eitherthe single-shot or double-shot mode of the Py-GC/MS while the ther-mally assisted hydrolysis and methylation (THM-GC/MS) and thedouble-shotmode of Py-GC/MSwere used for the analysis of theunagedand aged alkyd samples.

The pyrograms of the binders that had been mixed with pigmentsdid not display any difference in the Py-GC/MS results in comparisonto the pyrograms obtained for the pure binders. The Py-GC/MS resultsobtained from both aged acrylic binding media mixed with pigment,as well as those of the aged alkyd and PVAc mock-ups in comparisonwith the respective aged binders also did not show any change or for-mation of newpeaks, apart amore pronounced alteration of the bindersthemselves described in the following section.

The results achieved with the second-step pyrolysis of double-shotPy-GC/MS and single-shot Py-GC/MS show that the Plextol® D498 isbased on a co-polymer of n-butyl acrylate (nBA) and methyl methacry-late (MMA)while the Primal® AC33 is characterised by a co-polymer ofethyl acrylate (EA) andmethyl methacrylate (MMA) [10]. In addition tothe peaks of the main monomers, the pyrograms of each acrylic binderare also characterised by the formation of several and different oligo-mers (sesquimers, dimers, and trimers) at higher retention time (RT)[10]. Oligomers are molecules consisting of few monomer units and

generally formed under pyrolysis conditions in a lesser amount. Onthe other hand, the pyrolysis products of the polyvinyl acetate recordedin the single-shotmode of the Py-GC/MS and shown in Fig. 1c aremain-ly based on acetone (m/z= 43, 58), 1-3 cyclopentadiene (m/z=39, 66),acetic acid (m/z= 43, 60), benzene (m/z=52, 78), and toluene (m/z=65, 91) [12]. The thermal desorption step of the double-shot modeallowed the detection of the diethyl phthalate (DEP) type of plasticiserusually used in PVAc (Fig. 2c). The detection of the Di-tert-butyldicarbonate (DTBD) type of reagent is probably due to the impurity ofthe PVAc product, as it is also the case for octanone (Fig. 2c).

The main components of the alkyd binder—detected by thermallyassisted hydrolysis and methylation (THM-GC/MS) analysis—arebenzoic acid, methyl ester at RT 17.1 min (m/z = 51, 77, 105, 136),trimethyl ether of pentaerythritol at RT 19.7 min (m/z = 55, 69, 71,75, 85, 101, 114, 128), phthalic acid, dimethyl ester at RT 26.1 min(m/z = 77, 92, 133, 163, 194), and the fatty acids from oil such asmethylated fatty acid peaks related to azelaic acid (2C9:0) at RT27.7 min, palmitic acid (C16:0) at RT 34.6 min, oleic acid (C18:1) atRT 37.4 min, stearic acid (C18:0) at RT 37.8 min, and linoleic acid(C18:2) at RT38min. Furthermore, the intermediate oxidation productsof 2-octenal, nonanal, 2-decenal, 2,4 decadienal, 2-undecenal and thedryer compound 2-ethylhexanoic acid were identified in the thermaldesorption step of the double-shot mode of Py-GC/MS (Fig. 2b [11]).

After 83 days of UV ageing, the main evidence of photo-oxidativeprocesses on both acrylic binders observed in the thermal desorptionpyrogram of the double-shot techniques are the peaks of thesesquimers, dimers, and trimers, which were not registered in theunaged material (Fig. 2a). The thermal stability of the samples has evi-dently been changed by the photo-ageing processes leading to the pro-duction of oligomers already detectable during thermal desorption.Additionally, the most intense peak of n-butyl acrylate (BA) of Plextol®D498 and of ethyl acrylate (EA) of Primal® AC33 in comparison to themethyl methacrylate (MMA) peak decreased gradually after UV ageing,whereas the MMA peak became the most intense one (Fig. 1a). This re-duction in intensity of the EA and BA peaks shows a higher sensitivity tophoto-oxidation reactions of these monomers compared to a monomerwith lower structural units such as MMA. A further indication for aphoto-oxidation process concerns the octyl phenol polyethoxy ethanolsurfactant. No traces of octyl phenol previously detected in the thermaldesorption step in the double-shot mode were found anymore in thepyrogram of the aged Plextol® D498 and Primal® AC33 by plottingthe corresponding single ion mass profile atm/z= 135 [10].

After 83 days of UV ageing, one of the main signs of photo-oxidativeprocesses of the alkyd binder is shown by a decrease of unsaturatedfatty acids such as oleic (C18:1) and linoleic acid (C18:2) followed by adecrease of oxidation products (cis-9-hexadecenal, cis-9-octadecenal,and octadecanal) from unsaturated fatty acids and increasing of shortchain products (1-octene and hexanal). Additionally, an increase of dicar-boxylic acid such as azelaic acid (2C9:0) and suberic acid (2C8:0)was alsodetected. These results obtained by THM-GC/MS and second-step pyroly-sis of double-shot of Py-GC/MS (Fig. 1b [11]) suggest oxidation, cross-linking, and chain scission of unsaturated fatty acids, meanwhile dicar-boxylic acid are produced [6,14]. Other indications of the photo-oxidation of the alkyd binder are given by the total decrease ofthe group of aldehydes (2-octenal, nonanal, 2-decenal, 2,4-decadienal,2-undecenal) and the dryer compound (2-ethylhexanoic acid) priorydetected in the unaged binder by the thermal desorption step of thedouble-shot of Py-GC/MS (Fig. 2b), probably due to further oxidationor cross-linking of such products.

After UVageing, itwaspossible to observe an increase of the phthalicanhydride (PhA) peak in the thermal desorption step of the double-shotof Py-GC/MS (Fig. 2b). By calculating the peak area ratio of the phthalicanhydride in the thermal desorption step (PhA1) and in the pyrolysisstep (PhA2), it was possible to see an increase of the PhA1/PhA2 inthe aged alkyd, which indicates that more free ortho-phthalic acid wasformed during UV ageing [11]. The main pyrolysis product phthalic

Fig. 1. Pyrograms of unaged and aged acrylic Primal® AC33mixed with ultramarine blue (PB29) (a), alkyd (b) [11], and polyvinyl acetate (PVAc) mixed cadmium yellow (PY37) (c), ob-tained with the pyrolysis step of double shot Py-GC/MS. (Peaks: 1) Benzene, 2) 1-Octene, 3) Hexanal, 4) 2-Methyl-1-pentanol, 5) Heptanal, 6) Benzaldehyde, 7) 1-Decene, 8) Octanal,9) Octanoic acid, 10) Benzoic acid, 11) Phthalic anhydride, 12) 1 (3H)-Isobenzofuranone, 13) Pentadecane, 14) Benzeneacetic acid ethyl ester, 15) trans-9-Octadecenal, 16) Pentadecane,17) Tetradecanal, 18)n-Hexadecanoic acid, 19) cis-9-Hexadecenal, 20) cis-9-Octadecenal, 21)Octadecanal, 22) 2-oxo-octadecanoic acidmethyl ester, 23) 1,3-Benzenediol,monobenzoate,24) trans-2-Hexenyl benzoate [11]).

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anhydride of the alkyd is generally formed by the cleavage of a C\\Obond during thermal decomposition of the alkyds based on ortho-phthalic acid and the rise of phthalic anhydride indicates that freeortho-phthalic acid was formed during UV exposure. Therefore, the in-crease of PhA1/PhA2 detected by the double-shot Py-GS/MS in theUV-aged alkyd may suggest that the carbonyl phthalic ester undergoesphoto-oxidation reactions through Norrish type I reaction. It has beenreported that aromatic polyesters are prone to photolysis and photo-

oxidised by Norrish I photo-cleavage as main initiation step of photo-degradation [15–19]. Scheme 1 shows 2 different possibilities of cleav-age of the ester group creating 4 different primary radicals. The mecha-nism of photo-cleavage indicated in path I illustrates the formation ofcarboxyl and alkyl radicals (Ia). The next step (Ib) is based on the ab-straction of a hydrogen atom by the polymer carboxy radical from thepolymer chain to yield a phthalic acid end group, which under thesame mechanism of photolysis and photo-oxidation pathways of Ia)

Fig. 2. Pyrograms of unaged and aged acrylic Primal® AC33mixedwith ultramarine blue (PB29) (a), alkyd (b), [11], and polyvinyl acetate (PVAc)mixedwith cadmium yellow (PY37) (c),obtained with the thermal desorption step of double shot Py-GC/MS.

954 V. Pintus et al. / Microchemical Journal 124 (2016) 949–961

and Ib) may form free ortho-phthalic acid (1c and 1d). Likely themechanisms of photo-cleavage of path I, the path II displays the finalformation of free ortho-phtahlic acid (IId) through the initialproduction of acyl and alkoxy radicals (IIa) followed by oxidation

and abstraction of hydrogen (IIb), cleavage (IIc), and again abstractionof hydrogen (IId).

The photo-oxidation process also affected the chemical stability ofthe polyvinyl acetate (PVAc) binder. The amount of acetic acid—which

955V. Pintus et al. / Microchemical Journal 124 (2016) 949–961

was detected in the aged PVAc with both methods of Py-GC/MS—washigher than in the unaged ones indicating the degradation of the filmby the UV ageing procedure (Figs. 1c and 2c). The increase in intensityof the peak related to the acetic acid shows a growth of free aceticacid, which is in agreement with the Norrish type II mechanism basedon the formation of an excited carbonyl group following absorption oflight [20,21]. Furthermore, a decrease of the peak related to the DEPtype of plasticiser in the binder was observed after UV ageing (Fig. 2c),according to the slow evaporation of such plasticisers, resulting in em-brittlement and degradation of the paint film [22,23].

3.3. FTIR-ATR analysis of unaged and aged samples

The FTIR-ATR absorption bands of both acrylics—Plextol® D498 andPrimal® AC33—alkyd and polyvinyl acetates binders are summarised inTable 3. Some of the selected pigments used for preparing the mock-upsshow characteristic IR absorption bands, which in some cases overlapthe absorption bands of the binders. For instance, the C\\O/C\\C skeletalvibrations of the binders (1,250–900 cm−1) are masked by the ultrama-rine blue pigment by the overlapping Al, Si\\O4 asymmetric stretchingbands at 1,069, 986 with a weak shoulder at 1,095 cm−1. Similarly to ul-tramarine blue pigment, bone black pigment (C, Ca3(PO4)2, CaCO3)masksthe C\\O/C\\C infrared region of the binders with a strong peak at1,020 cm−1 and two less intense peaks at 1,086 and 960 cm−1 of thephosphate groups (PO4

3−). Moreover, the raw umber Cyprus pigmentbased on iron oxide Fe2O3 with water manganese oxide MnO2 has aband around 1,000 cm−1 of the iron oxides. On the other hand, the cad-mium red and cadmiumyellowpigments do not absorb in the consideredmid-infrared region (4000–370 cm−1).

Generally, the most prominent differences in the absorption bands ofthe unaged and aged binders were obtained for the acrylic and alkyd,while the polyvinyl acetate (PVAc) seemed to be themost stable. Normal-ly, the differences between the unaged and aged samples were moreprominent when the binders were mixed with the pigments. On theother hand, after 83 days of UV ageing, the acquired infrared spectra ofthe unaged and aged pigments did not show any remarkable difference.

Table 3ATR infrared absorptions of the acrylics Plextol® D498 and Primal® AC33, alkyd Medium 4, an

Bond type Acrylic Plextol® D498 Acrylic Primal®

p(nBA/MMA) p(EA/MMA)

(cm−1) (cm−1)

O\\H stretching – –C\\H stretching – –

2,955, 2,931, 2,874 2,949, 2,888, 2,2,852 shoulder –

C_O stretching 1,726 1,723C_C stretching – –C\\H bending – 1,513

– 1,466–1,4511,449, 1,436 shoulder –1,386 1,3811,361, 1,343 1,360, 1,343

C\\O and C\\C stretching – 1,2791,236, 1,159, 1,143 1,240, 1,149– –1,115 1,1091,063 1,0611,022 shoulder 1,021990 –963 964944 –

C\\H rock 842 843808 –– –754 759– –

The FTIR-ATR results of the unaged and aged samples are summarisedbelow and arranged according to the corresponding sample types,i.e. a) binders and b) mock-ups.

3.3.1. BindersBy considering the following infrared regions, it is possible to de-

scribe the UV degradation of all investigated binders:O\\H stretching (3,650–3,100 cm−1): A gradual broadening in this

infrared region, probably due to the formation of alcohol groups and/or hydroperoxidic structures [3], was shown by the UV-aged acrylicPlextol® D498 and Primal® AC33 binders (Fig. 3a). The acrylic samplesshowed a continuing increase in absorption as well as a band broaden-ing of the OH groups with a maximum peak around 3,495 cm−1 as waslikewise observed in the alkyd resin after the UV exposure (Fig. 3c). Thechanging of this infrared region is mostly due to the formation of newalcohol groups along the fatty acid portion through either ß-scissionsor Norrish type I reactions during the photo-oxidation degradation ina similar way to the natural and photo ageing of the binder simulatingindoor conditions [24–26].

Theunaged andUV-aged polyvinyl acetate (PVAc) binder andmock-ups did not show any type of absorption in the O\\H stretching regionbetween 3,650 and 3,100 (Fig. 4c).

C\\H stretching (3,100–2,800 cm−1): After 83 days of UV ageing, thesharp peak at 2,890 cm−1 of polyethoxylated surfactant of both acrylicswas no longer present according to the Py-GC/MS results (Fig. 3a).During UV exposure, the methylene groups of the alkyd binder wereprone to diminish possibly through Norrish type I and II reactions [27]or due to the oxidation of double bonds [27,28], which is shown by agradual decrease in absorption in the asymmetric and symmetric(C\\H)CH2 stretching at 2,926 and 2,855 cm−1, respectively (Fig. 3c).Theweak band of the aromatic_C\\H stretching at 3,070 cm−1 detect-ed in the alkyd binder did not change after 83 days of UV ageing.

By comparing the C\\H stretching region between 2,800 and3,100 cm−1 of the IR spectra of the unaged and aged PVAc bindingmedium, no significant changes in the weak absorption of the doubletof the –CH3 and –CH2 asymmetric stretching vibration at 2,973 and2,926 cm−1 were noticed (Fig. 4c).

d polyvinyl acetate Mowilith® 50 binders investigated.

AC33 Alkyd Polyvinyl acetate (PVAc)

Medium 4 Mowilith® 50

(cm−1) (cm−1)

3,495 –3,070 –

860 2,926–2,855 2,973, 2,926– –1,720 1,7291,600, 1,580, 1,507, 1,489 –– –1,465–1,451 1,433– 1,449, 1,4351,385 1,370– –– –1,254 1,225, 1,1201,174, 1,166 –1,1141,068, 1,040 –1,026 1,018– –974 –– –– 844– 795774 –– –740–709 –

Fig. 3. FTIR-ATR spectra of the acrylic Primal® AC33 and alkyd mixed with raw umberCyprus (PBr8), (a) and (b) respectively, and of the alkyd binder (c) before (solid line)and after UV ageing (pointed line).

Fig. 4. FTIR-ATR spectra of the acrylic Plextol® D498 [10], alkyd, and polyvinyl acetate(PVAc) mixed with ultramarine blue (PB29), (a) (b) and (c), respectively, before (solidline) and after UV ageing (pointed line).

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C_O stretching (1,750–1,700 cm−1): The difference of the carbonylstretching absorption and broadening band in all four different UV-aged binders are very similar.

The IR spectra of the UV-aged acrylic binders in comparison to the IRspectra of the unaged samples are characterised by a gradual reductionof the carbonyl group absorption at 1,726 cm−1, which may be relatedto the loss of the butyl group of the Plextol® D498 (Fig. 4a) and ethylgroup of the Primal® AC33. Although a loss of an ester group tookplace in the alkyd and PVAc binders during UV ageing with a conse-quent formation of the free ortho-phthalic acid and free acetic acid,respectively, the absorption of the carbonyl peak at 1,720 cm−1 in thealkyd binder as well as at 1,729 cm−1 in the PVAc binder remained un-changed (Figs. 3c and 4c). This is probably due to the compensation ofthe loss of the ester groupswith some new carboxyl products absorbingin the same region and formed by reaction of secondary macro radicalwith oxygen [4].

The broadening of the carbonyl stretching band of the acrylics andalkyd shows the formation of newly photo-degraded products such as

carboxyl acids, ketones, and aldehydes [24–27]. Particularly, the pro-gressive absorption at 1,773 cm−1 in the acrylics and between 1,600and 1,710 cm−1 in the acrylics (Figs. 3a and 4a) and alkyd (Figs. 3cand 4b) indicates respectively the formation of a γ-lactone structure [3,4,29] or open chain anhydrides [30] and the growing of unsaturatedmolecules with a C_C bond due to the polymer chain scissions [3] orketones products as a consequence of the reaction of oxygen moleculeswith radicals and followed by ß-scission [31]. In case of the PVAc binder,the weak broadening of the carbonyl peak in the range between 1,700and 1,600 cm−1 (Fig. 4c) suggests the increase of free acetic acid [32]after UV ageing according to the Norrish type II mechanism [33],corresponding with the results obtained by the Py-GC/MS analysiswith the double shot.

C_C of aromatic ring (1,650–1,450 cm−1): The two characteristicsharp and intense peaks at 1,600 and 1,580 cm−1 followed by two lessintense and weak peaks at 1,507 and 1,489 cm−1 of the aromatic C_Cstretching of thepolybasic acid part of the alkyddid not showany signif-icant change after UV ageing (Fig. 3c).

C\\H bending, C\\O and C\\C stretching, and C\\H rocking (1,450–700 cm−1): A gradual reduction of the absorption of the typical peaksof the PEG type surfactant at 1,343, 1,110, and 963 cm−1 was ob-served in both acrylic binders after UV exposure, which agrees with

957V. Pintus et al. / Microchemical Journal 124 (2016) 949–961

the Py-GC/MS results in which no detection of any traces of the surfac-tant found in the aged samples.

Where the doublet of the most intense symmetric CH2 bendingat 1,465 cm−1 and the less intense asymmetric CH3 bending at1,451 cm−1 was shown in the IR spectrum of the unaged alkyd binder,after UV ageing, the asymmetric CH3 bending of the doublet became themost intense one, complemented by an increase in absorption of thesymmetric CH3 bending at 1,385 cm−1 (Figs. 3b,c and 4b). The absorp-tion variation of the CH3 to CH2may be explained by the decrease of thefatty acid chain length and increase of short chain products in the agedalkyd binder due to the chain scission degradation of the unsaturatedportion of the fatty acid in the alkyd structure. This result agrees withthe higher amount of short chain products detected in the aged alkydsamples by the thermal desorption step of double-shot of Py-GC/MS. Aslight decrease of the C\\O stretching phthalic group at 1,254and 1,114 cm−1 was observed after UV ageing of the alkyd binder(Figs. 3b.c and 4b). As it has already been reported in the Py-GC/MSSection 3.2, the formation of the free ortho-phthalic acids from the car-bonyl phthalic ester (Schema 1) may explain the gradual reduction inabsorption of the phthalic C\\O stretching band due to the C\\O bondscission from the alkyd structure. Furthermore, after UV exposure ofthe alkyd binder, it was noticed that the peak at 974 cm−1 of the outof plane_CH bending of trans-structures gradually became a shoulder(Fig. 3c), mostly due to the oxidation of the double bonds [27,28] orpossibly through Norrish type I and II reactions [27]. No significantchange was noticed for the aromatic C\\H in plane deformation peaks

Schema 1. Suggested Norrish type I photolysis and photo-o

at 1,068 and 1,040 cm−1 aswell as the aromatic C\\Hout of plane bend-ing at 774, 740, and 709 cm−1 of the alkyd binder after UV exposure.

Similar to the other infrared regions of the PVAc binder/mock-ups,no noticeable difference could be determined in the wavelength regionbetween 1,450 and 700 cm−1 by comparing the unaged and UV-agedspecimens.

3.3.2. Mock-upsAll of the differences in the IR spectra recorded between the unaged

andUV-aged binders weremore pronounced in themock-ups, especial-ly when both acrylic binders weremixedwith the ultramarine blue pig-ment and when the alkyd binder was mixed with the ultramarine blueand raw umber Cyprus pigments. Particularly, the absorption bands ofthe ultramarine blue at 1,095, 1,067, 988, 688, 653, 580, and 441 cm−1

and of the raw umber Cyprus at 3,175, 1,016, and 890 cm−1 becamemore evident in comparison to the absorption bands of the binders(Figs. 3a,b and 4a–c),mostly due to the photo-degradation of the organ-ic part in the mock-ups. Furthermore, noticeable changes of the absorp-tion spectrum of Plextol® D498mixedwith ultramarine blue are shownby the progressively decrease of the C\\O stretching peaks at 1,144 and1,063 cm−1 and another peak at 846 cm−1 related to the C\\H rockingas well as the disappearing of the C\\O stretching peaks at 1,159 and1,114 cm−1 (Fig. 4a [10]). This effect related to the C\\O stretchingbands and to the C\\H rocking band may suggest the loss of an estergroup in agreement with the reduction of C_O absorption and to thefast oxidative chain scission of the polyethoxylated surfactantswhen ex-posed to UV light [34,35].

xidation pathways of carbonyl phthalic ester of alkyd.

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In contrast to all of the other pigments, most of the principal absorp-tions gradually increased when the cadmium red and particularly thecadmium yellow pigment was mixed with both acrylic binders as wellwith the alkyd corresponding to an increase of the ester side groups.

Similar to the FTIR-ATR results obtained on the aged polyvinyl ace-tate (PVAc) binder, no obvious differences could be determined by com-paring the spectra of the unaged and UV-aged PVAc mock-ups. Possiblydue to the low amount of the acetic acid formed during UV ageingthrough the Norrish type II mechanism (as shown by the results of thePy-GC/MSdouble-shotmethod) and complemented by the C_C double

Table 4Shifts in the L*, a*, b*, and E* coordinates of the acrylic Plextol® D498 (a) and Primal®AC33 (b), alkyd (c), and polyvinyl acetates (d) mock-ups as well as of the pigments(e) before and after UV ageing.

a)

Acrylic mock-up (Plextol® D498) ΔL* Δa* Δb* ΔE* 2000

Titanium white anatase 1.54 0.41 −0.41 1.20Titanium white rutile −0.49 0.12 −0.472 0.56Cadmium yellow −2.37 −3.56 −6.8 2.54Cadmium red 1.83 −0.68 −4.9 2.63Hydrated chromium oxide green 3.01 6.11 0.55 3.63Ultramarine blue −5.71 1.76 0.98 4.23Raw umber Cyprus −0.54 −2.43 −3.87 3.68Bone (ivory) black) −0.76 0.07 0.22 0.58

b)

Acrylic mock-up (Primal® AC33) ΔL* Δa* Δb* ΔE* 2000Titanium white anatase 2.31 0.44 0.32 1.59Titanium white rutile 0.83 0.11 −0.2 0.57Cadmium yellow −1.28 −1.59 −9.03 2.00Cadmium red 0.64 −1.46 −2.96 1.22Hydrated chromium oxide green −6.16 0.00 1.54 4.78Ultramarine blue 3.07 0.18 −3.09 2.65Raw umber cyprus 0.8 −1.64 −2.71 2.56Bone (ivory) black) −1.67 0.05 0.34 1.20

c)

Alkyd mock-up ΔL* Δa* Δb* ΔE* 2000Titanium white anatase −4.02 −0.43 −2.53 3.33Titanium white rutile 2.08 0.45 2.26 2.57Cadmium yellow −0.86 −1.68 2.70 1.44Cadmium red 3.67 −6.81 −7.23 4.50Hydrated chromium oxide green −4.08 4.37 2.48 4.70Ultramarine blue 13.12 −28.00 37.36 16.01Raw umber Cyprus 11.66 −6.74 −10.84 12.68Bone (ivory) black) 1.25 0.08 −0.20 0.92

d)

PVAc mock-up ΔL* Δa* Δb* ΔE* 2000Titanium white anatase 1.29 0.12 −3.74 3.41Titanium white rutile 0.33 0.16 0.64 0.65Cadmium yellow −1.43 0.43 −0.17 1.08Cadmium red −1.01 −0.01 0.77 0.96Hydrated chromium oxide green 1.78 −2.56 1.34 2.19Ultramarine blue 1.91 −2.94 0.64 1.98Raw umber Cyprus 1.83 −0.44 −0.88 1.59Bone (ivory) black) −1.28 −0.02 0.05 0.90

e)

Pigments ΔL* Δa* Δb* ΔE* 2000Titanium white anatase 3.00 −0.10 −3.30 3.38Titanium white rutile 4.65 0.43 −1.63 3.14Cadmium yellow 6.43 −6.75 −1.71 5.58Cadmium red 3.18 0.50 −0.22 2.94Hydrated chromium oxide green 0.86 3.02 −0.65 1.34Ultramarine blue 1.64 −0.24 −0.71 1.44Raw umber Cyprus 4.46 −1.82 −1.33 3.98Bone (ivory) black) −1.07 0.39 1.63 1.84

bond in the main chain, no variations were detected in the infraredspectra of the UV-aged PVAc mock-ups.

3.4. Colour measurements of unaged and aged samples

The colourmeasurement results of all investigatedunaged and83-dayUV-aged acrylics (Plextol® D498 and Primal® AC33), alkyd, and PVAcmock-ups and pigments are summarised in Table 4, which includes theshift in the values of the lightness/darkness (L*), redness/greenness (a*),yellowness/blueness (b*), and total colour (E*). Additionally, the CIELABdiagrams shown in Figs. 5 and 6 highlight the pattern change of a* andb* values after UV ageing, indicating colour changes, while in the leftside of the figure it is possible to see the differences in L* values corre-sponding to the brightening and darkening of the paint films.

The ΔL*, Δa*, Δb*, and ΔE* values recorded for the investigated pig-ments show a slight difference in the shift of the L*, a*, and b* coordi-nates as well change colour (Table 4). The general behaviour of mostof the analysed pigments was given by a slight increase in L* valuescorresponding to a brightening of the pigment during the UV exposure.Additionally, the cadmium yellow pigment had the greatest ΔE* value(ΔE* = 5.58) based on the shift of L* of 6.43 and of a* and b* of −6.75and −1.71, respectively.

Generally, the differences in the shift of the L*, a*, and b* coordinatesbetween the unaged and UV-agedmock-ups aremore prominent in thecase of the pigments mixed with the alkyd binder, which shows thisbinder to be least stable when exposed to UV light. In particular, the ul-tramarine blue mock-up, and to a slightly lesser extent, the raw umberCyprus mock-up, had the greatest shift of a* (Δa* = −28.0) and b*(Δb* = 37.36) between unaged and aged samples showing a strong re-duction in red and blue, respectively. The ultramarine blue alkydmock-up had the highest shift of L* (ΔL* = 13.12) towards the brightening.The brightening of the surface could be explained by smoothing of thefilm surface during UV exposure [4].

The acrylic Plextol®D489 andPrimal®AC33mock-ups showed sim-ilar colour change results, where the ultramarine blue, raw umberCyprus, and the hydrated chromium oxide green had the greatest shiftin the values of a*, b*, and L*. In contrast, the colour results for thePVAc mock-ups changed substantially less than for the alkyd and bothacrylic mock-ups after UV exposure. This latter result agrees with theFTIR-ATR results.

Additionally, all four type of binders mixed with the titanium whiteanatase were shown to be slightly more sensitive to the UV light thanthe titanium white rutile according to the more pronounced differenceof the a*, b*, and L* values after UV ageing (Table 4),which is completelyin accordance to the literature [36].

4. Conclusions

The UV photo-oxidative stability—including the UV-B range forsimulating sunlight outdoor conditions—of four different bindingmedia such as acrylics in two different co-polymer forms (Plextol®D498 and Primal® AC33), alkyd, and polyvinyl acetate (PVAc)when mixed with different types of inorganic pigments, particularlyused in modern and contemporary art, was investigated throughthe single-shot and double-shot Py-GC/MS, FTIR-ATR, and colourmeasurements.

After UV exposure, these techniques recorded different ageing be-haviours of the acrylics, alkyd, as well as polyvinyl acetate (PVAc). Thedouble-shot technique of Py-GC/MS was especially effective at detect-ing the effect of the photo-oxidative reactions in a more precise and de-tailed manner relative to the single-shot method. Depending on theanalysed binder, the comparison between the thermal desorption ofthe double-shot technique results obtained from the unaged and agedsamples shows main evidence for photo-oxidative deterioration.Concerning both acrylic bindingmedia, the detection of dimers, trimers,and sesquimers can be detected at a lower temperature than the normal

Fig. 5. CIELAB diagram of the acrylic Plextol® D498 and Primal® AC33, alkyd, and polyvinyl acetate mock-ups investigated before and after UV ageing showing the shifts of their L*, a* andb* coordinates.

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temperature used for the pyrolysis. This can be considered an effect ofthe UV light on the investigated materials, showing the impact of UVon their thermal stability. Additionally, the total decrease of the non-ionic surfactant after UV exposure, which was possible to detect in theunaged binders only by plotting its mass profile in the thermal desorp-tion pyrogram,was sign of the sensitivity of the acrylic binders to theUVlight. The pyrolysis step of the double-shot Py-GC/MS, as well as thesingle-shot Py-GC/MS, showed the higher sensitivity of the BA and EAmonomers of the Plextol® D498 and Primal® AC33, respectively, incomparison to the MMA monomers by the gradual decrease of the BAand EA peaks in the pyrograms of the aged binders. On the other

hand, newly formed products including unspecific aldehyde, lactones,and acidic oxidation products were detected by FTIR-ATR analysisfrom both acrylic binders.

The results obtained from the thermal desorption of the double-shottechnique of the unaged and aged alkyd binder showed a higheramount of phthalic anhydride indicating that more free ortho-phthalicacid was produced during the UV ageing, which is shown by FTIR-ATRby the gradual reduction in absorption of the phthalic C\\O stretchingband due to the C\\O bond scission from the alkyd structure. This resultis particularly important because it is the first time this phenomenonhas been detected during photo-oxidative studies of alkyd binders

Fig. 6. CIELAB diagram of the pigments investigated before and after 83 days of UV ageing showing the shifts of their L*, a*, and b* coordinates.

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and especially because the formation of free ortho-phthalic acid wasinduced by the effect of UV light including the UV-B range. Furthermore,the decrease of unsaturated fatty acid and increase of saturateddicarboxylic acid was detected by THM-GC/MS and the pyrolysissecond step of the double-shot method. In the thermal desorptionstep of double-shot Py-GC/MS of the aged polyvinyl acetate (PVAc)binder, the detectable amount of acetic acid increased, while theamount of the DEP type of plasticiser highly decreased in the PVAcaged samples. This latter result was also evinced by FTIR-ATR by thebroadening of the carbonyl peak.

Apart from more accentuated differences already noticed betweenthe unaged and aged binders, the results obtained with the double-shot and single-shot Py-GC/MS did not show any difference betweenthe unaged and aged mock-ups, whereas the FTIR-ATR results demon-strated that each pigment induces to each binder a different ageing be-haviour. For instance, the cadmium yellow pigment mixed with bothacrylics and alkyd seems to promote the formation of ester side groupsduring UV exposure, which corresponds to an increase of all principalinfrared absorptions in the IR spectra. Additionally, the colour

measurements demonstrated the higher sensitivity to the UV light ofthe alkyd in comparison to the acrylics and PVAc mock-ups showedby the bigger shift of the main coordinates L*, a*, b*, and E* coordinates.Thus, the UV ageing of different types of binders either alone or whenmixed with different inorganic pigments shows that their differentchemical composition has a first order effect on their resistance to UV-radiation.

In this work, the Py-GC/MS technique, especially the double-shotmode, proved to be suitable for the photo-oxidative ageing studies ofacrylics, alkyd, and PVAc binders, whereas the FTIR-ATR method andcolour measurements provided useful information about the influenceof inorganic pigments on theUV stability of the analysed bindingmedia.

Acknowledgements

This work has been funded by Regione Sardegna (Italy), “ProgrammaMaster and Back anno 2009” Alta Formazione and the Austrian ScienceFund, Project no. L699-N17. We thank Anthony J. Baragona (Institute of

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Science and Technology, Department of Conservation Science, Universityof Applied Arts, Vienna, Austria) for helping with the English corrections.

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